Seriously if anyone thinks Manchester united managed to grab a huge win because Mourinho is not their manager anymore has no idea how football works.
If a team wins because they lack a manager thats not the managers problem thats combination of couple things, first of all when a manager leaves team has to show what they got so when the new one comes they can get a spot in the starting line up so everyone plays much better, when you have a manager and they have a proffered starting line up those people know they got the job so they lay back, that is a big difference.
Secondly when manager leaves the team is working hard to prove that the trouble was the manager and they are actually good so they play extra well in order to defend themselves and put the blame on others. ManU will go back to early season ways soon but it will just take couple of games like this and couple games with new manager, after that its downhill again.
I think it was due to the devils mismanagement.
They did even mentioned before the match started yesterday Pogba and Mourinho relationship was very very bad.
You can see this on his twitter under "Caption this!" picture after he was fined for posting it.
And if you are not getting along with the best player on the roster (well their highest paid player anyways) then you should be replaced not the player since he is the one on the pitch playing with his teammates and does not have necessarily an issue with them.
The manager was the constant problem the team was having. That was the issue with them.
They lost their matches under Mourinho with small margins. They replace him and they score the most goals in a game since being under Alex Ferguson in 2013.
You can't deny that is just a coincidence.
I believe it was the toxic environment that Mourinho was creating for himself that caused his departure even before the season was over and the club didn't care if it was a sacking before the holiday season even started and did it to him out of spite.