It was a case of so bad to so good in a little over 30 minutes as the Red Devils scored three times in 13 minutes.
West Ham played their opponents off the park in the first-half and deservedly went ahead when Tomas Soucek tapped in at the back post.
The London Stadium faithful, 2,000 strong, saw their side create several chances but failing to take them ultimately cost them.
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Ole Gunnar Solskjaer introduced Bruno Fernandes and Marcus Rashford at the break.
The latter got United's third after Paul Pogba and Mason Greenwood scored in quick succession to turn the game on its head.
Solskjaer's side, for now at least, are just two points off the summit.
Here are five talking points from an entertaining game in the capital.
1. Comeback kings strike again
When United trudged off the pitch at half-time the commentator curse struck.
We were reminded that United have come from behind to win in four of their last six away games.
So the scene was set after Soucek opened the scoring for the Hammers following a lackluster display from United.
It took just three second-half minutes for the visitors to turn the contest on its head as Pogba and Greenwood struck.
There wasn't the same drama we got at St Mary's last Sunday, but this was typical United.
Solskjaer's side appear to have a Jekyll and Hyde personality when it comes to games on the road, how long they can keep it up for may come to define their season.
2. West Ham's reality check
The Hammers have been going quietly about their business but were dealt a blow on Saturday night.
When they lost against Newcastle on the opening day things appeared bleak.
But they have rebuilt since and before the game with United only in 2015/16 had they registered more points at this stage of the season.
In the first-half, they by far outplayed Solskjaer's side but perhaps a lack of clinical edge told.
Failure to get a second goal meant United were always in the game and, despite their haphazard form, showed that the visitors are where West Ham want to be.
3. Consistently inconsistent
Being a United fan must be one of the most entertaining yet painful rollercoasters about.
Even in last weekend's dramatic win at Southampton, there were signs of frailty.
Regardless of results, eventually, performances that regularly produce errors will catch up with you.
Solskjaer has regularly sought consistency from his side, but he is yet to find it despite seeing them pick up three points once again.
Winning can cover a lot of cracks, but it is better they are addressed rather than ignored.
The Norwegian named the same back four for the fifth consecutive game so you'd think some sort of solidity is being established - far from it.
Defensively they were all over the place with the Hammers spurning several first-half chances while Soucek was unmarked to notch the opener.
Gary Neville was bemused at how his former side continue to deceive and keep you guessing during commentary.
"They just can't produce consistency this Manchester United team, the squad," he said.
"There's no rhythm from week to week. You think 'OK we're getting somewhere then next week you think 'Ooh, back to square one'."
Yet United were sensational in the second-half and it continues to amaze you how they can produce such contrasting showings in the same game.
4. Henderson's rude awakening
The United goalkeeper has had to be patient this season having returned to Old Trafford following several years away.
His opportunities have been few and far between, but when they've come they haven't been kind to him.
Henderson has been playing behind a solid Sheffield United defence for the past two years.
Now he is having to deal with a backline that allow Demba Ba half a pitch with which to score.
And things didn't get much easier at West Ham where Henderson was regularly exposed.
On several occasions, the shot-stopper saw Hammers players bearing down on him.
His accumulation of clean sheets may not be well served to play behind United's defence, but it is certainly a platform with which he can prove his worth to Gareth Southgate.
5. Fan-tastic news
Fans were allowed to attend a Premier League game for the first time in 271 days on Saturday evening.
Leicester's win over Aston Villa at the King Power back in March was the last time fans were present at a top flight game.
West Ham's London Stadium holds 60,000 but despite only being around three per cent full the atmosphere was noticeable.
The hosts ran out to a genuine cry of 'Forever Blowing Bubbles' which proves that fans are well and truly on their way back and after 38 minutes they were cheering a goal.
Some unsavoury scenes across the capital have threatened to grab the headlines, but for the most part the return of fans is a huge and positive step back to normality.