Mourinho is a very volatile but circumstantially great manager. Were I in charge of United 3 years ago I wouldn't have touched him with a 10-foot-pole. Yet, he's been by far, the most capable manager you've had post Red Nose.
Ownership though - screws you.
You can change managers and players as much as you want - but until ownership wants to win the league and *backs* a manager 100% - United are screwed.
Players will figure out that the manager isn't backed all the way and some players will accordingly become lazy and entitled. Other players will be in it for themselves - and not for the team. If the manager isn't backed 100% they'll get away with this.
Typically - winning sides care about winning above all else to a man - trainers, players, managers and ownership are all in accord. There's a collective sense of "us" and "we" - and in a winning side, "we" want to win. In a twist of irony, "United" are anything but, "united."
At present, Manchester United are a blueprint for failure.