For the Hammers were exceptional against the Red Devils. Tactically more astute. Defensively more organised. And offensively more threatening. The 3-1 victory was exactly what they deserved.
Felipe Anderson opened the scoring after just five minutes and Andriy Yarmolenko's deflected strike put the Irons in control at half time.
Marcus Rashford did reduce the deficit in the second period but three minutes later Marko Arnautović ensured the closing stages would be comfortable for the home side.
The defeat puts further pressure on José Mourinho, whose side have collected just ten points from their opening seven top flight fixtures. It's the worst start the Red Devils have made to a Premier League campaign.
Of most concern will be the performance as United were lethargic throughout, defended poorly and lacked ideas in attack. Rashford's goal, a neat back-heel flick from a corner, was the only moment of real quality displayed by a player in Red.
So for Mourinho, desolation. For Manuel Pellegrini, vindication. And it's with the Chilean that we begin the five things we learned from West Ham's victory.
Time was all Pellegrini needed
After the opening four games of the Premier League season West Ham were bottom of the table and there was talk Pellegrini, who was only appointed in the summer, would be sacked.
The knee-jerk reaction was somewhat laughable given the money the Irons spent in the summer on new recruits, many of whom were still finding their feet in East London.
And in games five, six and seven, Pellegrini has proven himself more than capable of leading West Ham into the top ten of the English top flight.
His side won at Everton, held Maurizio Sarri's Chelsea to a goalless draw and have now dispatched Manchester United with ease.
Those results are impressive but the performances more so as the Irons have shown different qualities in each. Against United it was their poise on the ball that stood out.
Yes, United were poor, but the Hammers used the ball smartly – Mark Noble a standout – and their front three, Arnautovic, Anderson and Yarmolenko caused constant problems.
The signs are certainly positive for West Ham. What a difference a few weeks can make.
Mourinho's 5-3-2 fails
During his reign at Old Trafford Mourinho has toyed with a back three; a system he'd never used on a consistent basis prior to taking charge at Manchester United.
Results with the formation have usually been mixed, which is why it hasn't become a default shape for the Red Devils.
But at the London Stadium the back three returned with versatile midfielder Scott McTominay on the right side, Chris Smalling in the centre and Victor Lindelöf. Eric Bailly was only a substitute.
The defensive shape was curious given the opposition, but how United started the game was plain concerning. They sat extremely deep, on the edge of their own penalty area, and didn't press the ball.
It meant West Ham were able to settle into the game and gain confidence in possession. And that enabled the Irons to take an early lead.
The build-up to Anderson's goal was excellent; Mark Noble's brilliant through ball sliced open the United backline; Pablo Zabaleta's cross was inch perfect; and the Brazilian's flick finish was sublime.
The opener arrived in the fifth minute and, strangely, there was no reaction from Mourinho's men for over 20 minutes. They let West Ham dictate the game, with Declan Rice constantly open and able to play passes to either flank.
Eventually the Red Devils' defence moved 15 yards further up the pitch and the Irons midfield was closed down. But it wasn't an effective press and West Ham were still able to keep possession comfortably and eventually add to their lead.
Mourinho persevered with the system until ten minutes after half time, where he brought off Lindelöf, introduced Marcus Rashford and returned to a 4-2-3-1 formation. Not that it ultimately helped United produce a comeback.
Impressive Irons backline shine again
This summer many a West Ham fan would've been forgiven for hitting Google when the Irons announced the signings of Fabián Balbuena and Issa Diop. The former came from Corinthians while the latter joined after impressing for Toulouse.
But the France Under-21 international was brought into the side to partner Balbuena against Arsenal and they've started alongside each other in the four games since.
And, on the basis of their display against Chelsea and now Manchester United, the duo are forming an excellent partnership.
Against the Red Devils, Balbuena made a game-high ten clearances, three tackles and two interceptions. Diop, meanwhile, registered two tackles, two interceptions and five clearances.
The only cross they didn't deal with was the corner that led to Rashford's goal.
Frustrated Pogba must be trusted
There was a time when Paul Pogba and Mourinho quite liked each other, or at least were civil.
When the Frenchman returned to Old Trafford in 2016, he said about his new manager: “I am really looking forward to working with Jose Mourinho.” The Portuguese, meanwhile, stated Pogba would be “a key part of the United team I want to build here for the future”.
A little over two years on, though, and the pair's relationship has completely broken down.
Initially it was framed through cryptic quotes from both sides. Now, however, there's no hiding the fact that Mourinho does not enjoy the status Pogba has at Old Trafford: “No player is bigger than the club. If I am happy with his work he plays, if I am not he doesn't play.”
This week at least it appears Mourinho was happy with United's record signing's effort as he was included in the starting XI.
Pogba was used on the left of the midfield three, alongside Maroune Fellaini and Nemanja Matic. The creative burden placed upon him.
But, as discussed above, the system didn't work. United were too sluggish in possession, which allowed the Irons to get back into their defensive shape, and that meant Pogba was stifled.
In the first half alone the French superstar collected the ball on several occasions, looked up to play a pass but had few options to hit.
His frustration became evident in the second period and Pogba started to play desperate passes in an attempt to get his side back into the game. The majority didn't come off and after 70 minutes he was brought off for Juan Mata.
The Frenchman was poor, there's no doubt about that, but he at least took responsibility. Others in this United team hid, not wanting the ball for fear of making an error.
And Pogba's forceful personality, that you sense Mourinho doesn't like, is what needs to be used for good. The 25-year-old needs to be trusted with the keys to the United midfield, and perhaps even the side.
He has the quality to get United back on track, Mourinho must show the humility required to give it to him.
Sânchez omission curious
There is no denying that since he arrived at Manchester United in January Alexis Sánchez has been hugely underwhelming.
He is, accordingly to several reliable reports, the best paid player in the Premier League. So with just three goals in 23 appearances, clearly the Chilean isn't doing enough.
Yet the change to 5-3-2 should've been an opportunity for Sánchez to impress. Mourinho deployed the system regularly in pre-season and the former Arsenal man looked sharp in a central attacking role. Those games in the U.S, although uncompetitive, were the best United have seen of the 29-year-old.
But Sánchez wasn't in the starting XI at the London Stadium. Even more strangely, he wasn't even included in the squad despite having travelled down to the capital with the rest of the Red Devils' travelling party.
When asked about United's No.7's omission, Mourinho simply replied: “Options, just options.”
Given his form this season, Sánchez could do little to argue against being dropped. However, being left out of the match-day 18 was another big call from the United boss, who already has strained relationships with several members of his side.