Villa's goal was somewhat fortunate, in that the corner should've been a goal kick and De Gea was interfered with. And certainly you don't expect Bruno to pull a Roberto Baggio from the spot in a critical moment.
However, the result is 100% fair. United were just not good, even taking into account the forced subs due to injuries.
Ole has done a great job rebuilding the United culture and improving the squad. However, he has shown little to no tactical growth over his tenure and even with vastly superior attacking players at his disposal, there is no discernible attacking system aside from "let them go out and make something happen". Even with the best players you need some sort of plan to break down a stubborn defense, even the lesser teams are very well drilled defensively these days. He has shown far too much loyalty to his coaching staff: why does Mike Phelan — under whose assistance United played some pretty dire stuff during Fergie's era — remain in his job?
The schedule over the next couple of months in Europe and domestically is going to be absolutely brutal, and they're going into it having lost three out of four. I don't see Ole surviving at this point in time, and it pains me to say that. He's taken the club as far as he can, and he now looks like he's well out of his depth.