[5] On 9 October 2007, Mérida was ordered by the courts to pay a fee around €3.2 million to Barcelona as compensation for his "failing to meet the terms of a personal pre-contract" with the club.

On 3 April 2008, he extended his deal with the club by signing a long-term contract, being expected to play an important role in the future as manager Arsène Wenger described him as "an absolutely amazing player".

[10] On 18 December 2007, Iñaki Badiola, prospective presidential candidate of Real Sociedad, claimed he agreed a loan deal for Mérida until the end of the season in Segunda División.

"; he made his Premier League debut the following day – hours before his 19th birthday – coming off the bench for Samir Nasri in the dying minutes of a 3–1 win at West Bromwich Albion.

He replaced Aaron Ramsey midway through the second half of the team's first match, against Atlético Madrid, and started and created a goal for Eduardo against Rangers, a 3–0 win which certified the tournament's conquest.

[18] On 28 October 2009, in that season's League Cup, Mérida contributed to a 2–1 home defeat of Liverpool, scoring from the edge of the area with the ball hitting the post on its way past goalkeeper Diego Cavalieri.

"[24] Mérida made only eight official appearances for Arsenal during the season, including one against Bolton Wanderers on 17 January 2010 in which he scored his first Premier League goal in a 2–0 victory.

[25] On 25 May 2010, Atlético Madrid officially announced that Mérida was on his way to the Vicente Calderón Stadium on a free transfer, after refusing Arsenal's contract offer at the end of the campaign.

[26] He scored his first official goal for his new club on 27 October in a 5–0 win against Universidad de Las Palmas CF in the Copa del Rey,[27] repeating the feat in the second leg (1–1 draw at the Vicente Calderón Stadium).

[28] On 16 December 2010, Mérida scored his third goal for the Colchoneros, netting in his first touch in a 1–1 draw at Bayer 04 Leverkusen, with the defending champions of the UEFA Europa League crashing out after the group stage.

[38] A week later, in the year's Copa do Brasil, he was sent off for two bookings as a late substitute against Sport Club Internacional in the first game of the quarter-finals,[39] being unused in both legs of the final, a 3–1 aggregate loss to Flamengo.

[44] On 3 December 2015, Mérida scored a penalty as his team beat top-flight club Villarreal CF 3–2 at home in the last 16 of the Spanish Cup,[45] but in an eventual 4–3 aggregate defeat.

[57] In the next group game he continued his fine form and netted twice in a 2–0 win over Nigeria;[58] after resting against Venezuela he played 70 minutes in the last-16 tie against Italy, which finished with a 3–1 loss.