But the fixtures are coming thick and fast for Mourinho's men.
Having made the mammoth trip to Rostov for the first leg of their Europa League last-16 tie on Thursday, United welcome the Russian outfit to Old Trafford three days after they face Chelsea, before travelling to Middlesbrough in the Premier League on Sunday.
With United struggling to crack the top four domestically, the Europa League presents a golden opportunity to return to Europe's elite club competition, with the winners earning a place in next season's Champions League.
Speaking exclusively to Omnisport, United great Neville said: "It comes down to what Mourinho prioritises. The FA Cup could be a competition that United sacrifice because they have to try and get into the Champions League.
"Obviously the only two routes are the Europa League and Premier League games.
"It will be a big challenge mentally on Monday. It might be the one, at this moment in time, that they can sacrifice in terms of trying not to complicate the fixture congestion any more."
Despite enjoying a 17-match unbeaten run in the Premier League, United have been unable to improve on sixth position, with their title ambitions all but over for another season.
After being frustrated in a 1-1 draw against Bournemouth last time out, Mourinho's men find themselves 17 points adrift of leaders Chelsea and three off the Champions League qualification spots.
And Neville - who won eight Premier League titles and two Champions League finals during a trophy-laden career at Old Trafford - concedes Mourinho's task of returning the club to its former glory is bigger than initially anticipated.
"I thought United would have a chance at the start of the season," Neville said. "But when Diego Costa and Eden Hazard turned up and they corrected their defensive problems by going three at the back, it became pretty apparent quickly that the old Chelsea were back.
"There's a lot of Premier League winners in that Chelsea team. The issue has been you don't know which Chelsea will turn up from one season from the next. Once the real Costa and Hazard turned up and you had David Luiz in defence and [N'Golo] Kante in midfield, you have a championship-winning team, or the makings of one.
"There is still a bit to go, they [Chelsea] still have some work to do. It won't be as comfortable as we think, there might be some dropped points in the next few weeks.
"The challenge at United was probably bigger than what everyone thought. When I look at it now, I don't think anyone could've won the league with United this season."