Aged 15, Valdez left his home and family to move to Ypané when he joined the Youth Academy of Atlético Tembetary, and eventually made his first-team debut in the Paraguayan División Intermedia in 2000. The second division club were not able to pay him a salary large enough to earn a living and his family were not able to provide funding for him. Valdez confessed the difficulty he faced when leaving his impoverished family and his home to begin playing with the club, for two years he lived under the wooden plank grandstands of the stadium in a stairwell, using a makeshift bed of blankets and cardboard which laid in the stairwell beneath the stand. He never told his parents, who were poor land workers. Valdez stated, “For two years I lived under the grandstands, we were 12 or 13 people and some of them only endured two or three weeks.” While living 250 kilometres away from his family, Valdez had no home while playing for Tembetary. He worked night shifts in a sawmill to pay for food and drink. He was a 16-year-old exposed to danger and the Paraguayan climate, boiling hot summers and freezing cold winters, which Valdez described as “hell, frightening and horrible. But if you have nothing, what can they steal? I had nothing to give”. Jürgen Born, chairman of the board or management of German Bundesliga club Werder Bremen and who worked for Deutsche Bank in Latin America, stated that he saw a “madman who never stopped running”, remembering youth games in Ypané. He bought Valdez a ticket to Germany and got on the phone to Werder Bremen. In December 2001, Valdez flew over to Germany for a trial. Valdez failed at first, not impressing in his trial but the club president’s wife, who was a fellow Paraguayan, asked for Valdez to be given another trial. He scored four times in his first trial match and was offered a contract to join Werder Bremen II, the club’s Regionalliga Nord team.

Upon joining Werder Bremen in December 2001, Valdez did not know a word of German and knew just enough Spanish due to being much more fluent in Guaraní. Valdez stated that his first months were very hard because of the cold weather and that he did not know how to order food properly. For the first few months, he did not eat anything except chicken and chips because he would see it through the glass at a restaurant.

In 2003, Valdez rejected the chance to play for the Germany national under-21 team in favour of Paraguay, having been invited to trial with Germany U21 by coach Jürgen Kohler, he was eventually was selected to represent the Paraguay under-20 national team at the 2003 FIFA U-20 World Cup in the United Arab Emirates. Prior to this, Valdez had been unheard of from his under-20 teammates and coach, Rolando Chilavert, brother of José Luís Chilavert, and was selected due to his presence in German football. Valdez did not know the names of any of his teammates from the squad and eventually met them on the airplane. Paraguay’s U20 squad had already finished third at the 2003 South American Youth Championship, and eventually qualified for the FIFA U20 World Cup. Valdez was one of three players based outside of Paraguay, alongside Dante López at Israel’s Maccabi Haifa and Víctor Hugo Mareco at Italian club Brescia. The squad also featured players such as Édgar Barreto, Julio dos Santos, Ernesto Cristaldo and Erwin Avalos, with several of these players going on to represent Paraguay U23 at the 2004 Summer Olympics. Valdez featured in all three group-stage fixtures against United States, South Korea and Germany, scoring Paraguay’s second goal against Germany in their 2–0 victory, which resulted in Paraguay finished in second position of their group with six points. During the competition, Valdez humorously revealed to Paraguayan newspaper ABC that in the fixture against Germany U20, he faced his Werder Bremen teammates Christian Schulz and Alexander Walke who made a bet with him that if he would score a goal then one of his teammates would owe him a dinner and would pay for his meal. Valdez then featured in Paraguay’s round of 16 clash against Spain U20, where which Paraguay were beaten 1–0.

In 2003, during an interview with Paraguayan newspaper ABC, it was reported that Valdez was an admirer of Ronaldo and Roque Santa Cruz. Valdez also stated that grew up being a fan of Primera División Paraguaya club Cerro Porteño and that his idols were Francisco Arce and Carlos Gamarra, when the club were crowned champions vía penalties in 1994.

Valdez scored his first goal for the senior Paraguay squad in a game against El Salvador on 17 August 2005. During the 2006 FIFA World cup qualification campaign, Valdez scored after 64 minutes in a 1–0 away win against Venezuela on 8 October 2005. Paraguay finished fourth with 28 points as Valdez was selected for the final 23-man squad for the tournament, where he was handed the number 18 shirt.

At the 2006 World Cup, Valdez started in Paraguay’s opening Group B clash against England, which they lost 1–0. After a solid performance from Valdez against Sweden in the second match, Paraguay would lose 1–0 after 89 minutes and be eliminated from the tournament. Valdez would play 66 minutes of Paraguay’s final group stage fixture against Trinidad and Tobago, which they won 2–0, as he was replaced by Nelson Cuevas.

On 9 September 2009, in Asunción in the round 16 of the qualifiers, Valdez scored after 27 minutes of the first half to give Paraguay a 1–0 lead against Argentina; Paraguay won the game 1–0 and qualified for the 2010 World Cup. Paraguay finished the campaign in third spot, behind Brazil and Chile, with 33 points as Valdez had scored five times in 17 appearances.

In 2009, Valdez once, gun in hand, chased off thieves stealing his car and on another occasion ran into his blazing home to save his dog. On 23 December 2009, it was reported that upon returning home with his wife in the early hours of the morning, they found their house in flames as a fire eventually destroyed their house. No member of their family was hurt, however, the family suffered the loss of their Golden Retriever dog named Arami, that Valdez had gifted to his wife. Months before this incident, however, in the same house, burglars intended to steal Valdez’s Mercedes-Benz and Valdez himself had faced the burglars with a shotgun blast in order to prevent the theft.

In August 2010, Valdez signed a three-year contract with Hércules after admitting that he had received offers to play in Russia. Valdez became the most expensive signing in the history of Hércules having been transferred for €3.5 million. His debut resulted in a brace against Barcelona on 11 September in a surprising 2–0 victory at the Camp Nou. Valdez scored his second double for Hércules in a 3–1 win against Levante on 29 November. He would then score in a 4–1 against Atlético Madrid on 11 January 2011. In January, when Valdez had scored six times in 11 appearances, it was reported that Real Madrid had considered signing him when Madrid striker Gonzalo Higuaín was ruled out for the season following back surgery, however a transfer was never established. He received a red card on 61 minutes in his second last game of the 2010–11 season in a 3–2 loss against Racing de Santander on 8 May.

Valdez was not included in Paraguay’s squad for the 2007 Copa América in Venezuela, however a few months later, he took part in the 2010 World Cup qualifiers. He opened his goal scoring account in round two of the qualifiers scoring in a 1–0 win against Uruguay on 7 October 2007. Paraguay were undefeated until round six where they had lost 4–2 against Bolivia in La Paz; Valdez scored his third goal of the qualifiers.

In August 2011, Valdez signed a four-year contract with Russian Premier League side Rubin Kazan. His debut in the 2011–12 season came in a 3–1 away loss to Krasnodar on 10 September, where Valdez received a yellow card on 27 minutes. He made his first appearance in the 2011–12 UEFA Europa League on 15 September in a 3–0 victory over Ireland’s Shamrock Rovers. Valdéz scored his first goals in the Europa League, a double, in a 4–1 home win against Shamrock Rovers on 30 November. Two weeks later, he again scored in the Europa League in a 1–1 away draw against PAOK on 15 December. This result meant that Rubin Kazan would progress to the 2011–12 UEFA Europa League knockout phase, where Rubin lost 2–0 on aggregate to Olympiacos.

Valdez was selected in the squad for the 2011 Copa América where Paraguay had been drawn in Group B with Brazil, Ecuador and Venezuela. During a friendly fixture several days prior to the Copa América, Valdez had scored his 11th national team goal in a 2–0 home victory against Romania on 11 June 2011.

In Paraguay’s opening group stage fixture on 3 July 2011, Valdez was substituted onto the field on 73 minutes for Lucas Barrios as Paraguay drew 0–0 with Ecuador. On 9 July 2011, Paraguay faced Brazil when Valdez was substituted onto the field on 56 minutes for a yellow-carded Barrios while Paraguay were drawing 1–1 with Brazil. Valdez then scored a goal on 66 minutes as Paraguay took the lead to 2–1. Brazil eventually levelled the score on 89 minutes as the game finished at 2–2.

Valdez scored his first league goal for Rubin in a 1–1 away draw against Zenit Saint Petersburg on 25 March; Valdez was substituted onto the field in the 46th minute and equalised for Rubin in the 58th minute before being substituted off of the field in the 68th minute. Two weeks later, he scored his second league goal in a 1–1 away draw against Dynamo Moscow on 7 April. He made his first appearance in the 2011–12 Russian Cup in Rubin Kazan’s 2–0 semi-final win against Rostov on 11 April. In the final game of the season, Valdez opened the scoring for Rubin as they fought back from a goal down to defeat CSKA Moscow 3–1 on 13 May 2012.

On 8 July 2012, it was reported that Argentine Primera División club Newell’s Old Boys had announced their intention to sign Valdez with the signing of former Paraguay national team head coach Gerardo Martino in charge of the team for the 2012–13 season. It had also been reported that Valdez had stated that he was interested in transferring to Newell’s Old Boys. On 14 July 2012, Valdez was substituted onto the field for Obafemi Martins in the 64th minute of Rubin’s 2–0 2012 Russian Super Cup victory against Zenit. The win meant that Valdez had claimed his second title at the club within just under 12-months since his arrival. It was then reported that Spanish La Liga club Espanyol and Primera División Paraguaya club Libertad were interested in signing him.

Valencia purchased Valdez’s contract outright from Rubin Kazan in February 2013; he subsequently signed a contract with the club lasting till June 2015.

On 19 July 2013, Valdez joined Abu Dhabi-based club Al Jazira for a fee believed in the region of €3 million. Wearing the number 9 shirt, Valdez made his first appearance for Al Jazira in a 1–0 away victory against Al-Shaab in a 2013–14 UAE Arabian Gulf Cup tie on 3 September 2013. Valdez made his 2013–14 Arabian Gulf League debut against the same team in a 2–1 away victory on 15 September, playing a full 90-minutes of the fixture. On 25 October, he scored his first league goal for Al Jazira in a 1–1 away draw against Al Dhafra. His goal came in the 38th minute.

On 8 January 2014, Valdez scored a double against Al Shaab in his side’s 3–0 league victory at home. His goals came in the sixth and tenth minutes of the first half, in a fixture which proved to be Valdez’s last appearance for Al Jazira as he was ultimately sanctioned and fined for his celebration. After a fan had whistled at Valdez, in response he scored his first goal and put his finger on his mouth to silence the fans and then scored his second goal and celebrated his goals by touching his crotch as a sign of having “balls”. The TV image had censured Valdez’s private part and Al Jazira stated that they were prepared to preserve the good behaviour of the sport and had decided to suspend Valdez for his gesture until further notice. Valdez had received the suspension immediately after the match and could not justify his behaviour in an interview. Moments after the match, Valdez wrote in English via his Twitter account that his objective was never to offend anyone instead to make understand that he uses his balls when he is on the field and due to the bad luck in his last games he could not score goals, he expressed it in this manner. Finally, Valdez expressed that during the rumours of his exit from the club that his intention was to stay at the club. Valdez was ultimately loaned to Greek outfit Olympiakos for the remainder of the 2013–14 season.

On 30 January 2014, Valdez joined Superleague Greece side Olympiacos on loan from Al Jazira, being issued with the number 18 shirt. He made his debut with the club on 2 February in a 2–1 home victory against Panetolikos, coming onto the field as a substitute for Marko Šćepović in the 60th minute. Two weeks later, he scored his first goal for the club in a 4–2 home victory against Platanias on 15 February. The goal came in the 68th minute and was assisted by Javier Saviola. Valdez’s second league goal for Olympiacos came in a 2–0 home victory against Panthrakikos on 15 March; he scored in the 35th minute via an assist by Alejandro Domínguez. Valdez participated in the 2013–14 Champions League round of 16 stage against Manchester United. In the first-leg, he replaced Hernán Pérez in the 86th minute, coming on to the field as a substitute for the bench in Olympiacos’ 2–0 home victory on 25 February. In the second-leg on 19 March, Valdez again entered the field for Pérez in the 57th minute was Olympiacos were defeated 3–0 and ultimately 3–2 on aggregate. On 23 March 2014, he scored a hat-trick in a 1–4 away win against Ergotelis. Valdez’s hat-trick all came in the space of ten minutes from his first goal, scoring in the 3rd, 9th and 13th minute of the first-half. He netted his last goal of the season in a 1–0 home victory against Aris on 30 March; the goal came in the 84th minute and was assisted by José Holebas. Valdez concluded the Superleague Greece season with six league goals in ten appearances. Olympiakos finished in first place of the 2013–14 Superleague Greece table and qualified for the 2014–15 Champions League. He made his first appearance in the 2013–14 Greek Cup on 2 April 2014 in a semi-final 2–1 first-leg home victory against PAOK, playing the full 90 minutes of the fixture.

On 29 July 2014, it was reported that Valdez had signed with Eintracht Frankfurt for the 2014–15 Bundesliga season, returning to the Bundesliga four years after departing the German competition in 2010. Valdez, who was issued with the number 11 shirt for the club, stated that his motivation for joining Frankfurt was because of being offered to join the club by coach Thomas Schaaf, who had coached Valdez during his years at Werder Bremen. Valdez mentioned that he had always maintained contact with Schaaf and believed that he was one of the few people that he felt he owed something to: “He made me a professional. For me, it was clear that I was going to go to Frankfurt when I received the offer. It is an enormous pleasure to return to the Bundesliga.”

Valdez played a big part in Paraguay’s 2014 World Cup qualification campaign, featuring in 11 qualification fixtures as Paraguay failed to qualify for the competition. Preceding a 2–0 defeat against Colombia in October 2012 which saw Paraguay bottom of the qualifiers table with only four points, Valdez responded in anger to Paraguayan fans that criticized the squad for their results: “I appreciate with my whole heart those few who continue to support us. In change, I lament the rest. We come here for shit money and they treat us like this? From one day to another, they forget about all the joy we gave them.” It was during Paraguay’s frustrated 2014 World Cup qualification campaign that Valdez informed Paraguay.com that the unsuccessful campaign was the price that the national team was paying for the replacement of coaches and transition of players since the 2010 World Cup. Valdez referred to the new base of the national team of not capturing what the Albirroja represents and that there were players who were still immature and did not know what it meant to be there representing the national team.

In February 2014, it was reported that Valdez insisted on ways to financial help former national teammate Salvador Cabañas, who had suffered economic issues since being shot in the head in January 2010, and it was ultimately decided that Valdez would organize a charity match scheduled for June 2014 which would feature the likes of David Trezeguet and Javier Saviola in order to raise funds for Cabañas.

Upon Valdez’s return from injury, he was substituted onto the field in the 74th minute in a league fixture against SC Paderborn on 14 March 2015 and scored for Frankfurt in the 82nd minute as his side won the fixture 4–0.

Valdez signed with Seattle Sounders FC of Major League Soccer (MLS) on 7 August 2015 as a Designated Player. He scored a goal in his debut for the club on 16 August, earning the second goal in a 4–0 win over Orlando City SC at CenturyLink Field, Seattle.

On 28 May 2015, Valdez was included in Paraguay’s 23-man squad for the 2015 Copa América by coach Ramón Díaz. During a friendly fixture against Honduras on 6 June 2015, Valdez received his first expulsion for the national team after he retaliated having gone to header the ball but was instead hit in the left side of the chest with the studs of Honduran player Maynor Figueroa, who had lifted his leg high enough to make contact with Valdez. Valdez exited the field of play lifting his shirt to reveal for himself the mark that had been left from the player’s studs and then applauding the crowd in appreciation for their attendance. Valdez then publicly apologized and stated that surely it would not occur again.

It appeared that Valdez was ineligible to participate in Paraguay’s opening group stage fixture against Argentina, however this was not the case. Valdez was in the starting XI in their opening fixture on 13 June 2015. He scored the Paraguay’s first goal from approximately 25 yards in their 2–2 draw against Argentina in La Serena as Paraguay recovered from a 0–2 deficit. After the draw, Valdez received the MasterCard Man of the Match award. It was reported that initially the award was given to Lionel Messi, who declined it, and it was also reported that Argentina had rejected the opportunity for another Argentine player to receive it, before the award was given to Valdez.

Colombian Categoría Primera A clubs Independiente Medellín and Atlético Nacional showed interest in the player and reports suggested that Valdez was to depart Seattle Sounders. On 18 January 2016, Valdez publicly responded to reports about his arrival to Medellín: “I’m proud to be a Sounder and very happy in Seattle. Don’t want to raise somebody’s hopes or disappoint my fans!”

Valdez’ first competitive appearances came in the home and away legs of the 2015–16 CONCACAF Champions League quarter final stages against Mexican club América; on 23 February 2016, Valdez played 73 minutes of the 2–2 draw in Seattle before being substituted off of the field. In the return leg on 2 March 2016, Valdez played a full 90 minutes of the fixture which concluded 3–1 in favor of América and ended 5–3 on aggregate. His contract ran until December 2016.

During the 2016 MLS Cup Playoffs, Valdez scored his only two goals of the season, helping Seattle advance to the MLS Cup, where they won their first league championship.

On 4 January 2017, Valdez signed with Primera División Paraguaya club Cerro Porteño.