While on holiday in the off-season, Muntari picked up a knee injury playing beach football that could keep him out for the entire first half of the 2012–13 season. On 20 February 2013, Muntari scored a goal to give Milan a two-goal lead against Barcelona ahead of the second leg in the last 16 tie in the UEFA Champions League. However, it was not enough as Barcelona knocked Milan out in the return leg at the Camp Nou, 4–0 and 4–2 on aggregate.