The defeat leaves United 14 points adrift of leaders Arsenal in the Premier League table, and the Portuguese believes that the reigning champions should now focus on Champions League qualification this campaign.
Mourinho told reporters: "It is hard, they are 14 points behind Arsenal, 13 behind Manchester City and 12 from us.
"Maybe one team collapses but for three to collapse is difficult, so for them I think it is the fight to finish in the top four.
"They will do that with their pride and tradition and fight and hopefully they win against the other contenders because now we do not play against them in the league."
The Red Devils host Sunderland at Old Trafford on Wednesday for the second leg of their Capital One Cup semi-final - which they trail 2-1 on aggregate - before resuming league commitments by welcoming Cardiff City to the North-West next Tuesday.