In 2008, his contract expired and RCD Espanyol signed him. He made his senior debut in the 2009–10 season, playing four games with the B team and being relegated from the third division.

In June 2010, Tello moved back to FC Barcelona B, who competed in the second level. On 9 November 2011 he made his debut for Barça’s main squad, playing the full 90 minutes in a 1–0 away win against CE L’Hospitalet in that season’s Copa del Rey; he scored a brace in the second leg, a 9–0 demolition.

Tello made his first La Liga appearance on 28 January 2012, coming on as a substitute for Adriano in the 75th minute of a 0–0 away draw at Villarreal CF. The following weekend, in his first league start, he opened the score in the eighth minute of a 2–1 home victory over Real Sociedad after a through ball by Lionel Messi, and played the entire match.

On 7 March 2012, Tello replaced Andrés Iniesta early into the second half of a match against Bayer 04 Leverkusen at the Camp Nou, taking his bow in the UEFA Champions League: he scored his first goal within two minutes, after Cesc Fàbregas played him through over the top and he ran from the halfway line to net from just inside the penalty area for the 4–0; six minutes later, from a similar position, he made it 6–0 for the hosts in an eventual 7–1 win (10–2 on aggregate).

Even though his future initially seemed uncertain, Tello managed to convince new manager Tito Vilanova and stayed in roster for 2012–13, inclusively going on to start in several games. On 19 September 2012 he scored the opener and assisted Messi in the second goal, in an eventual 3–2 home defeat of FC Spartak Moscow in the Champions League group stage, where he was also chosen as Player of the match.

On 13 December 2012, Tello signed a new contract with Barcelona, keeping him with the Blaugrana until June 2016. Nine days later he scored after only 30 seconds on the pitch, in the last play of a 3–1 away win against Real Valladolid.

Tello was a member of the Spanish squad that participated in the 2012 Summer Olympics in London, playing all three games in an eventual group stage exit. He made his debut with the full side on 14 August 2013, starting in a 2–0 friendly win in Ecuador.

On 16 July 2014, Tello was loaned to FC Porto of the Primeira Liga in a two-year deal. He scored his first goal for the club on 25 November, entering as a substitute for Ricardo Quaresma and concluding a 3–0 victory at FC BATE Borisov for the Champions League group stage. Five days later, he netted his first league goal, the opener in a 5–0 home win over Rio Ave FC.

Tello scored all of his team’s goals as they beat Sporting CP 3–0 on 1 March 2015.

On 22 January 2016, Barcelona and ACF Fiorentina reached an agreement for the loan of Tello until 30 June 2016. He made his Serie A debut on 3 February, starting a 2–1 win over Carpi F.C. 1909 at the Stadio Artemio Franchi, and scored his first goal 18 days later in a 3–2 victory at Atalanta BC.

On 16 August 2016, the loan was renewed for the upcoming season.

On 30 June 2017, Tello joined Real Betis for €4 million (potentially €5 million).