Arsenal kept their top-four hopes alive and ended Manchester United's 25-match unbeaten run in the Premier League with a 2-0 victory at the Emirates on Sunday.
United made eight changes from their Europa League semi-final first leg against Celta Vigo on Thursday, and they fell behind shortly after half-time as Granit Xhaka's long-range shot deflected over David de Gea and into the net.
Danny Welbeck doubled the lead three minutes later and Arsenal comfortably held on to give Arsene Wenger his first competitive win over Jose Mourinho.
The victory also keeps Arsenal in the hunt for a top-four finish, although they still need either Liverpool or Manchester City to drop points.
The Gunners are seven points behind Liverpool with two games in hand, while defeat is United's first in the league since October 23, and leaves them four points outside the top four with three games left to play.
Arsenal 2-0 Manchester United Xhaka⚽ Welbeck⚽ . . Manchester United top four chase suffer major blow at the Emirates. Meanwhile Arsene Wenger finally has his 1st premier league win over Jose Mourinho It could be a key one in the quest for top 4
United did have their top-four fate in their own hands after Liverpool's draw with Southampton earlier in the day, but Mourinho, perhaps with one eye on the Europa League return leg with Celta Vigo on Thursday, opted to rotate his starting line-up at the Emirates.
James Milner missed a penalty as Liverpool gave up the initiative in the race for the Premier League's top four after a 0-0 draw with Southampton at Anfield on Sunday.
The key moment of a largely drab game came in the 65th minute when Saints goalkeeper Fraser Forster denied Milner from the spot.
A frantic finish did not return a winner for either side, and Liverpool now again need other results to go their way in the run-in if they are to qualify for next season's UEFA Champions League.