I actually think United are overachieving. Mourinho's teams of past have traditionally had exceptional starting elevens. Like Pep, he's at his best with elite players and getting them that winning mentality.
I mean, if you consider his two stints at Chelsea, and his stints at Inter and Real Madrid you see just how good his players were.
This is the starting eleven of the weakest (of the four) title winning sides, Chelsea 14/15:
Courtois; Ivanovic, Terry, Cahill, Azpilicueta; Matic, Fabregas, Oscar; Hazard, Willian, Costa.
Given I've just called this his weakest previous side, you can see just how far behind United are. How many of this United team would get into this one? De Gea, Pogba and Matic (technically anyway).
The fact is, his players are nowhere near good enough. Excluding Bailly all his defenders are extremely limited. Martial and Rashford are promising but nothing more yet. The rest (e.g. Lingard) would have already been sold. In fact remember he sold two of his current United players whilst at Chelsea, Mata and Lukaku.
The only way this United side are going to seriously be a threat in the future is if they make a load of high quality signings, not just decent squad players. Fortunately for us, I find that they have a habit of signing just as many duds as they sign talents, so it'll take probably at least two summers for them to get to the necessary level. And Mourinho will almost certainly be gone by then, probably leaving the club in a mess in the process.