London: Juan Mata was sent off as Manchester United's hopes of Champions League qualification were dealt a serious setback in a 1-0 defeat at West Bromwich Albion on Sunday.
The Spaniard received two quick-fire bookings mid-way through the first half, for blocking a free-kick and then taking a swipe at Darren Fletcher, and West Brom made their numerical advantage count in the 66th minute when Salomon Rondon netted his fourth goal in six Premier League games.
It ended United's recent revival of four straight wins and left Louis van Gaal's side in sixth place, three points below the Champions League berths having played a game more than fourth-place Manchester City.
"We have played 60 minutes with 10 against 11 and this is the fifth match in 14 days, so it's very difficult to play against 11 players," said United manager Van Gaal, who branded Mata's first booking "stupid".
"That's the reason that we lost this game."
United are now in danger of being caught by forthcoming Europa League opponents Liverpool, who are three points behind them with a game in hand following a last-gasp 2-1 win at Crystal Palace.
"It's a big blow because these kind of matches, you have to win," Van Gaal added.
Asked what United require from their final nine league games to finish in the top four, the Dutchman replied: "27 points."
Mata was shown his first yellow card in the 24th minute at The Hawthorns for blocking Fletcher's free-kick and saw red two minutes later for clumsily swinging his foot at the former United midfielder.
Craig Dawson almost put West Brom in front immediately, only for his ambitious 25-yard lob to fall inches wide of the left-hand post.
Having survived until half-time, United started the second half strongly and threatened through young forwards Marcus Rashford and Jesse Lingard.
But West Brom claimed the points when Venezuelan striker Rondon gathered Sebastien Pocognoli's drilled left-wing cross before slamming home left-footed to lift Tony Pulis's men 15 points clear of the bottom three.
'Big moment' for Liverpool
"The most important thing is the next game," said Pulis, whose side are now one point shy of the 40-point mark traditionally synonymous with survival.
"As you've seen this year, results have been off the radar. Let's get 40 points. I think that's definitely safe."
Earlier, substitute Christian Benteke won and scored a 96th-minute penalty as 10-man Liverpool came from behind to win 2-1 at Palace.
Joe Ledley's 48th-minute strike and Liverpool midfielder James Milner's second-half dismissal for two bookable offences looked to have set Palace on course for a first win in 12 league matches.
But a slip by Palace goalkeeper Alex McCarthy gifted Roberto Firmino a 72nd-minute equaliser, before Damien Delaney's rash challenge on Benteke enabled the Belgium striker to net the winner from the spot.
"You can try (to make changes), but the rest is passion," Liverpool manager Jurgen Klopp told Sky Sports at Selhurst Park.
"Football is like this. You make a plan at the end, but the lads did great. This is the third league win in a row. I can't believe how it feels -- really good, a big moment for us."
Liverpool climbed two places to seventh after recording a third successive league win for the first time in a year.
Palace manager Alan Pardew, whose side are without a league win in 2016, criticised referee Andre Marriner's decision to award Benteke a penalty.
Delaney pulled out of a sliding challenge on Benteke inside the box, but the Irishman had already gone to ground and replays showed that his right knee had clipped the Belgian's left foot.
"(Benteke) is looking for it. Our player is pulling out of it," said Pardew whose team side remain nine points above the relegation zone.
"If the linesman gives it for the touch of the knee in the last minute, I think it is harsh."