United are facing the prospect of finishing outside the top four of the Premier League for the second consecutive season - they finished fifth under Louis van Gaal in the previous campaign.
And victory allowed Arsene Wenger's men to end United's 25-match unbeaten run and close the gap on their rivals to two points, with a game in hand.
Before Celta Vigo's visit to Old Trafford for the semi-final second leg on Thursday, Mourinho said his decision to rest key players in the league was a result of those circumstances.
"It is the first time I've left Highbury or the Emirates and they are happy". When we lost these players in a period of 17 matches in seven weeks, it was the impossible job. They played from the first minute until the last minute. Just that. Apart from that, I can not demand more from players that didn't play one minute of football in the last seven weeks. "We're just delighted for ourselves and the fans as well, they deserved to go home with a smile on their face". It was a long-distance shot, not even a short-distance shot. For me football is pretty much the same everywhere, the ball is round'. They scored, they won.
He also hailed 19-year old Axel Tunazebe who made his EPL debut.
"I don't think so, for two reasons".
"The kid [Axel Tuanzebe], the same".
"I didn't sign here for five years (to not improve as a player)". Both youngsters appear to have a future at the club, but it would be a surprise if some of the others on display did. "It's up to us, we need some luck along the way, but if we win the next four games, I'm convinced that we can do it".
Sunday's win ended the hoodoo and earned Arsenal only their 13th league win in 50 outings against the Red Devils. He did fantastically well to play 60 minutes and the goal is a lucky situation. But they were fantastic. "We have such a small squad at the moment, it's so good that these guys have a fantastic answer".
Arsenal striker Danny Welbeck is hoping their 2-0 win over Manchester United can prompt a strong finish to the Premier League season. You think I enjoy a big club like Arsenal not winning trophies? "Of course it's a different role but so long as things are going well, why change?"
"I haven't thrown my toys out of the pram but I do want to play to help the team", he said.
United have struggled to break into the Champions League qualification places all season, spending most of the campaign in fifth or sixth spot in the Premier League, and Sunday's loss leaves them in a hard position.
"But, obviously, everything for Thursday".
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