I think he was slowly building something at United until they lost away to Huddersfield last season. Since then it's fallen away in stages. They lost the first derby of last season to fall 11 points behind in the league, they then went behind inside the first minute against Spurs in a game they needed to win to keep the title race going, then they lost at home to West Brom to give us the title.
I was listening to Guardian Football Weekly a few days ago and one of the panellists asked a very simple question, "Regardless of what you think of Mourinho, he's been there two seasons and they've spent a lot of money - what single tactic or style of play are United actually good at, or defines them?" and the answer is nothing. They have no identity, no system they can rely on defensively or offensively, no plan, no real direction as a club or as a team.
It's not all down to Mourinho - their board looks at the game in the wrong way, their transfer policy has been bizarre for years, their owners are still milking the club dry and preventing it from becoming what it's in position to be - but he's the first large piece of a big puzzle that needs to be solved before United can get back to challenging us. There was still some comedy value in him being there, though, and I'm sad they've seen sense.