The Spaniard received two quick-fire bookings midway 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 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.”
Anthony Martial’s tame effort after two minutes was all Van Gaal’s side could muster, whereas the hosts proved a threat from the outset.
Rondon’s directness made him a nuisance but it was his vision that almost undid United, with his ball to Craig Dawson headed wide in the ninth minute.
Saido Berahino’s scuffed attempt from a clever Craig Gardner header was blocked by the returning Chris Smalling as the home side continued to press, with the natives unhappy to see Daley Blind’s high foot on Berahino adjudged to have taken place outside the box.
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.
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.
“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.”
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