A calamitous error by David De Gea condemned Manchester United to a 1-0 loss against West Ham at the London Stadium which leaves their top-four hopes in jeopardy.
Said Benrahma's tame, long-range effort in the first half should have been a routine save for the goalkeeper but, despite getting a hand to it, he was unable to stop it from bobbling over the line.
Manchester United, beaten 1-0 by Brighton last time out and only a point ahead of fifth-placed Liverpool, appeared in control of the game before De Gea's mistake but could ultimately have lost by a greater margin, with the hosts dominating much of the second half as they took a huge step towards Premier League survival.
David Moyes' side created numerous chances and were also denied what looked a clear penalty on the stroke of half-time when Victor Lindelof's handball went unnoticed by on-pitch referee Peter Bankes and VAR Stuart Attwell decided not to intervene.
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The decision ensured Erik ten Hag's side remained in the game but West Ham withstood late pressure to claim a deserved victory which was Moyes' first over Manchester United since his spell in charge of the club. The result sees the Hammers move to 37 points, easing their relegation fears in 15th.
Manchester United, meanwhile, remain in fourth place and, while they still have a game in hand on Liverpool, their stuttering form has thrown the top-four race open.
How West Ham overcame Man Utd
Manchester United started the game positively but lacked accuracy with their shooting as Bruno Fernandes and Antony dragged efforts wide of Lukasz Fabianski's right-hand post.
Christian Eriksen spurned another chance, shooting over the bar when left in space to meet Antony's pull-back, while Marcus Rashford struck another angled effort against the outside of the post.
West Ham only threatened sporadically in the opening stages and had De Gea to thank for their opener, the Spaniard burying his head in the turf after allowing Benrahma's tame strike, after Luke Shaw had lost possession, to trickle into the net.
It was the under-fire goalkeeper's fourth error leading to an opposition goal in all competitions this season - the joint-most by any Premier League player along with Tottenham's Hugo Lloris.
Man Utd tried to muster a quick response but Antony's deflected long-range effort hit the wrong side of the post, and they were fortunate not to concede a penalty just before the break when Victor Lindelof's handball inexplicably went unpunished.
The momentum remained with West Ham, though, with the hosts pinning Manchester United back and creating a succession of chances in the second half.
Declan Rice, outstanding on his return to the side following illness, was at the heart of most of it, while De Gea continued to look nervous in the Manchester United goal, the Spaniard fortunate when Michail Antonio was adjudged to have fouled him before poking the ball over the line following a corner.
Tomas Soucek also had the ball in the net, only for his header to be correctly ruled out for offside. The Czech midfielder also struck the side netting with one effort, while Lucas Paqueta fired narrowly wide on another occasion as they continued to push for a second.
Manchester United did show some improvement in the latter stages, finally managing to subject West Ham to an extended period of pressure, but the hosts defended resolutely, with Fabianski also saving smartly from Rashford and substitute Anthony Martial.
The home team left the pitch amid a party atmosphere at the London Stadium, while Manchester United's dejected players trudged off knowing the top-four battle is far from over.
Moyes: Massive win and a huge step to safety
West Ham manager David Moyes:
"Huge step [towards safety]. To beat Manchester United in a run-in is a huge step. Getting three points against whoever is huge, but against Manchester United is massive for us.
"It was probably well deserved given how well we performed. The players showed great character, great spirit. We had to dig deep at times but we also had great moments and played a lot more like the West Ham we've seen in the last couple of years.
"It's a great result going into the semi-final of a European competition and the positive we will take from it, playing against as good a side as Man Utd, we knew were up against it.
"Now we've got another big challenge in midweek and it's one we will looking forward to."
On being denied a penalty for Victor Lindelof's handball: "Just add it to the other ones which have happened to us in recent games. We hope that somewhere along the line in the coming games we get a penalty kick or something that goes in our favour."
Ten Hag: We want De Gea to stay
Man Utd head coach Erik Ten Hag on whether he still trusts David De Gea:
"We want him to stay and we want him to extend his contract.
"He has the most clean sheets in the Premier League. We would not be here in this position without him. He has my belief. No concerns.
"That happens, but I think as a team you have to deal with it, you have to show character and be resilient and bounce back.
"I think we started well. We dictated the game. We created chances. But we didn't take them. And then, one mistake. But even then, until half time, I think we played a decent game.
"In three days, twice an individual mistake leads to us losing games. I'm disappointed but it is what it is.
"Now we have one week [off]. It's the first time since Christmas that those players have a full week. We have to reset, reload, to get the energy back and then keep going."
What's next?
West Ham host AZ Alkmaar in the first leg of their Europa Conference League semi-final on Thursday. Kick-off 8pm. They then resume their relegation battle at Brentford on Sunday May 14 with kick-off at 2pm.
Manchester United have four games remaining to secure a place in the top four. They host Wolves on Saturday at 3pm, then head to Bournemouth on May 20 before finishing with two home games against Chelsea and Fulham.
West Ham's remaining fixtures
May 11: AZ Alkmaar (H) - Europa Conference League, kick-off 8pm
May 14: Brentford (A) - Premier League, kick-off 2pm
May 18: AZ Alkmaar (A) - Europa Conference League, kick-off 8pm
May 21: Leeds (H) - Premier League, kick-off 1.30pm
May 28: Leicester City (A) - Premier League, kick-off 4.30pm
Man Utd's remaining fixtures
May 13: Wolves (H) - Premier League, kick-off 3pm
May 20: Bournemouth (A) - Premier League, kick-off 3pm
May 25: Chelsea (H) - Premier League, kick-off 8pm, live on Sky Sports
May 28: Fulham (H) - Premier League, kick-off 4.30pm
June 3: Manchester City (N) - FA Cup final, kick-off 3pm
Man Utd's misery - Opta stats
- Manchester United have lost eight away games in the Premier League this season, their joint-most away defeats in a single season in the competition, along with 2021/22 and 2015/16.
- West Ham have ended a run of five consecutive league defeats against Manchester United, defeating them for the first time since September 2019 under Manuel Pellegrini.
- Each of Manchester United's last eight defeats in the Premier League have come away from home; only once have they had a longer such run in the competition - 10 between February 1995 and October 1996.
- Manchester United have won just two of their nine away games in the Premier League in 2023 (D2 L5), failing to score in four of their last six such matches.
- Manchester United have lost each of their last four Premier League matches in the month of May, failing to score a single goal in those games.
- West Ham manager David Moyes has defeated Manchester United in a Premier League match for the first time since August 2012 with Everton - prior to today, he had drawn twice and lost eight of 10 matches in the competition against them since the victory with Everton.