David De Gea made a huge goalkeeping howler as Manchester United suffered a 1-0 defeat at West Ham on Sunday that threatens to send their season into a tailspin.
Just a few days ago United were heavy favourites to finish in the Premier League's top four but now they are wobbling badly while the Hammers have edged closer to safety.
The result hinged on a catastrophic first-half error by De Gea, who let a tame shot from Said Benrahma squirm past him in the 27th minute after his team had largely dominated the opening period.
De Gea's blunder came on the day on which he became the most-capped goalkeeper in Manchester United's history, appearing for the 540th time to move ahead of Alex Stepney.
United, who suffered the pain of conceding a last-gasp penalty in their defeat at Brighton on Thursday, had been boosted by Arsenal's 2-0 win at third-placed Newcastle earlier in the day.
They are still in fourth place but are now just one point ahead of red-hot Liverpool in the race to qualify for next season's Champions League, even though they still have a game in hand.
A disappointed Erik ten Hag defended his beleaguered goalkeeper but said his team were still in control of their destiny this season with four league games left to play.
"Mistakes are part of football and in this team you have to deal with it and bounce back because it is a team sport," the United manager told BT Sport.
"Over the season he is the one with the most clean sheets and also we did it as a team. It can happen, it's football but everybody has to take responsibility."
West Ham, who play in the Europa Conference League semi-finals this week, are now seven points clear of the relegation zone.
United made a bright start at the London Stadium, with top-scorer Marcus Rashford hitting the outside of the post in the 17th minute.
But West Ham took the lead against the run of play when Benrahma's weak shot from distance somehow squirmed past the sprawling De Gea.
The Spaniard appeared to have it covered but his footwork did not look right and despite reaching the ball, his hand was not strong enough.
He lay prone on the turf while West Ham celebrated, unable to comprehend what had happened.
De Gea has had a mixed season -- he leads the race for the golden glove for most clean sheets in the Premier League but has also made some costly errors.
United hit the post again in the 32nd minute, when the lively Antony's powerful drive was deflected onto the woodwork and behind.
Ten Hag's men, who have just 49 goals this season -- the second-lowest tally in the top nine -- managed 11 shots in the first half but only one on target.
As the clock ticked down to half time, De Gea denied Benrahma a second before a huge penalty appeal for handball against Victor Lindelof, which was waved away by referee Peter Bankes.
West Ham had the ball in the net again early in the second half but the referee blew for a foul on De Gea by Michail Antonio.
United were rocking badly and Tomas Soucek headed the ball into the United net with less than 20 minutes to go but again the officials saved them, ruling the goal out for offside.
Ten Hag went for broke, throwing on Jadon Sancho and Marcel Sabitzer, having earlier introduced Anthony Martial for the ineffective Wout Weghorst but they could not find a way back.
West Ham boss David Moyes praised the "huge character of his side", saying it would be tough for teams below them to catch them, even though they are not mathematically safe.
"The number isn't confirmed that it keeps you in the Premier League so we have to look forward to the next game and try and pick up more points," he said.
