[1][2] On 26 March 2016, aged only 17, he made his senior debut with the reserves by coming on as a late substitute for Juan Cámara in a 2–0 Segunda División B away win against Llosetense.
[5][6] In February 2017, Mujica suffered a hamstring rupture which required surgery, which hampered his progress,[7] keeping him sidelined for nine-and-a-half months and missing out the club's promotion in the play-offs.
[10] He contributed with six goals in 16 matches for the side; highlights included scoring in the season's Copa Catalunya final in a 3–2 victory against UA Horta on 2 June, to help the club lift the trophy for the first time.
[17] A few days later, he played in the first team under manager Marcelo Bielsa in the club's first pre-season friendly, as a second-half substitute against York City in a 5–0 win.
On 30 May 2024, it was announced Mújica would join Qatari club Al Sadd SC at the opening of the transfer window on 1 July 2024, costing a fee of €10 million, and signing a four-year contract.