Assaidi was born in Beni-Boughafer, in northern Morocco, before his family moved to Amsterdam. He began his career with the AZ academy where he spent three years before leaving in 2006 to begin his professional career at Omniworld in the Eerste Divisie. After two seasons with the club from Almere, where he scored 3 goals in 36 games, he moved to De Graafschap to play in the Eredivisie, the top flight of Dutch football, in the summer of 2008. Assaidi spent just one year with the club before moving to Heerenveen on the last day of the transfer window. The move was seen as surprising because Heerenveen had said that they were not planning to buy any new players, but the performance of Assaidi at the beginning of the 2009–10 Eredivisie, scoring five goals in five games, meant that an exception was made to bring Assaidi to the club.

Under new manager Ron Jans, Assaidi became a key figure in the side. With two goals against his old club De Graafschap and Vitesse, combined with two assists against De Graafschap and NAC Breda, Assaidi was responsible for four goals in the first six games, the same number of goals that he created in his whole previous season. Assaidi had his best month in November 2010, when he scored in a 2–0 win against Excelsior, and delivered two assists against Heracles (3–2), 1 against Feyenoord (2–2) and 2 against Willem II (5–0). In December, Assaidi displayed a Man of the Match performance in a home match against defending champions Twente, where, despite an ankle injury, Assaidi led Heerenveen to a crushing 6–2 win over the champions by scoring the first hat-trick of his career, whilst also delivering two assists and winning a penalty. After the match, he was applauded for his performance by Jans, who said that Assaidi deserved a “10 out of 10 for his play”. His performances would later be rewarded with a first international call-up by Morocco national team head coach Erik Gerets for a friendly against Niger.

Assaidi was eligible to play for both the Netherlands or Morocco. His first invitation for a senior team came in February 2011, when Morocco national team coach Eric Gerets called him up for Morocco’s friendly match against Niger. Assaidi made his debut in Morocco’s 4–0 win over Niger, coming on as a substitute in the 77th minute for Adel Taarabt. Since it was a friendly, however, Assaidi was still eligible to play for the Netherlands. He announced on 14 February 2011 that his choice for Morocco was definite.

Assaidi scored his first Morocco goal on 4 June 2011 in a 4–0 2012 Africa Cup of Nations qualifier win against Algeria. In the final tournament in Gabon and Equatorial Guinea, he started Morocco’s opening game at the Stade d’Angondjé in Libreville, being replaced by Adel Taarabt at half-time in a 2–1 loss to rivals Tunisia. He took no further part as Morocco were eliminated from their group. Assaidi started two games in the next year’s edition of the tournament in South Africa, where his team fell at the same hurdle.

On 17 August 2012, English Premier League side Liverpool agreed a deal to sign Assaidi for an undisclosed fee thought to be in the region of £2.4 million. Liverpool manager Brendan Rodgers hailed the deal, saying that he was “delighted”, and described Assaidi as “an exciting player…that will excite the crowd”. He made his debut on 20 September against Young Boys in a 5–3 UEFA Europa League win, before registering an assist in a 2–1 win over West Bromwich Albion in the League Cup six days later. Due to impressive performances from young fellow wingers Raheem Sterling and Suso, however, Assaidi failed to break through to Liverpool’s regular starting line-up, making 12 appearances in 2012–13 of which four were in the league.

On 27 August 2013, Assaidi joined Stoke City on loan for the 2013–14 season. Speaking after joining Stoke, Assaidi revealed that he turned down other Premier League clubs after he held positive talks with Stoke manager Mark Hughes. He made his debut for Stoke in a 3–1 victory over Walsall in the League Cup on 31 August 2013. He played in the next round of the League Cup Tranmere Rovers after coming on as a 61st-minute substitute, he provided an assist for Peter Crouch to secure a 2–0 victory. He made his Premier League debut for Stoke on 5 October 2013, coming on as a 63rd-minute substitute in a 1–0 defeat against Fulham.

Assaidi scored his first goal for Stoke in a League Cup match against Birmingham City on 29 October 2013. On 7 December, he scored the winning goal in a 3–2 victory against Chelsea, which was Stoke’s first league win over the Blues since 1975. Assaidi scored a similar goal against Newcastle United on 26 December and also found the net against Everton on 1 January 2014. On 20 February, it was revealed that Assaidi suffered ligament damage during a club training camp in Dubai, ruling him out for up to six weeks. Upon returning from injury Assaidi scored in a 4–1 win against Fulham on 3 May 2014. Stoke manager Mark Hughes admitted that he would like to sign Assaidi on a permanent basis. Stoke, however, was unhappy with Liverpool’s asking price.

On 1 September 2014, Assaidi re-joined Stoke City on loan for the 2014–15 season. After making 11 appearances for Stoke, eight as a substitute, he was recalled by Liverpool in January 2015.

On 16 November 2014, Assaidi scored his first international goal for over three years, a long-range equaliser in a 2–1 friendly win over Zimbabwe.

On 15 November 2016, Assaidi was released by the club. Assaidi later admitted that he only signed for the club due to the salary, and that he was ready to return to the Premier League.

On 24 December 2016, Assaidi returned to the Dutch Eredivisie after signing two-and-a-half year deal with Twente.