Goals from Felipe Anderson, Andriy Yarmolenko and Marko Arnautovic saw Manuel Pellegrini's men run out deserved winners at the London Stadium with only Marcus Rashford offering any resistance in response.
Here are five things we learned:
1. United set up to fail
Cast against the backdrop of Paul Pogba's "attack, attack, attack" comments of a week ago it was hard not to wonder what the Frenchman privately made of Mourinho's choice of tactics this afternoon.
If you're being charitable Mourinho's decision to switch to a back three with midfielder Scott McTominay at the centre was confused. But in all honesty, it was a whole lot worse than that.
At its best a back three allows energetic full-backs to bomb forward, create overloads and expose the space afforded wide by a four-man defence. This version here was not that.
Romelu Lukaku was isolated. Pogba a non-factor. Martial even less effective than that. That Mourinho abandoned the entire experiment at the hour mark tells its own story.
Through the switch and the introduction of Marcus Rashford United were immediately more of a threat but the damage had already been done.
The fans are still behind him, at least the vocal away support here, but how long can this continue? This kind of non-event of a performance is fast becoming the norm.
2. United weak on the left
Luke Shaw's return to prominence has been one of the more positive stories of this young season. But the hype-train veered off the rails today as he was routinely exploited by Anderson and the overlapping (and inexplicably evergreen) Pablo Zabaleta.
It wasn't all on him - the less said about the increasingly hapless Victor Lindelof the better - but this was very much an afternoon to forget for the England man too.
Neither player seemed anywhere near comfortable in Mourinho's perplexing system and United were left horribly exposed as a result.
3. The West Ham way?
Whisper it quietly but this was actually very good from West Ham. Those four opening defeats feel a world away now with Manuel Pellegrini appearing to finally have found his preferred eleven.
There was some really good stuff on show too. The home faithful long for some open, attractive football and this was it, in parts at least, with signs that Pellegrini's attacking and aesthetically pleasing style is beginning to be put in place.
4. Not the West Ham way
West Ham have hardly been synonymous with strong and resolute defensive play in recent times - or ever really - but they were excellent here repelling everything United threw at them throughout the afternoon.
Goals the wrong way were a problem in that awful opening run but they've really tightened up since the international break. The win at Everton - with only a single goal conceded - followed up by the hard-fought draw against Chelsea and then today show a side transformed
Credit to Pellegrini and the new faces who've clearly been putting in the time behind the scenes.
5. Where's Alexis?
It's a fine question after Sanchez was left out of the matchday 18 for the game at the London Stadium. It's understood he was fit to play but just wasn't selected. That's quite the fall for a player so heralded on his arrival back in January.
It's hard to see a way back, short of a change of management, with others, even in an under-performing side, still producing more than the high-priced Chilean.
Where next feels even harder to predict.
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