Salomon Rondon's second-half strike was the only goal of the game as United struggled to enforce themselves on their opponents, particularly after going down to ten men.
The creative Spaniard Mata was sent off in the 26th minute after receiving two yellow cards, one which saw him jump in front of a West Brom free-kick, and the other a silly tackle on Darren Fletcher, meaning his teammates were forced to play over an hour with ten men.
Van Gaal disappointed
On the sending off United manager van Gaal said in his post-match interview: "The first was a stupid yellow card, you know it’s a yellow card in advance. The second you know you already have one so you have to take that into account."
Mata's sending off was the first of his career, and with that fact van Gaal thinks that the referee Mike Dean should have taken into account the player's character, saying that he "has to know the character of the players", which is why "an experienced referee is better, because he knows the players and the game."
Louis van Gaal found it difficult to criticise his players with a congested fixture list and having to try and win the game with one less man: he explained "It’s a big blow, these kinds of matches you have to win. We have played 60 minutes with ten men against 11 and this is the fifth match in 14 days."
Defeat into context
The Red Devils have a very important week to come as they travel to Anfield to face Liverpool in the last-16 of the Europa League on Thursday night, following that they have the small matter of an FA Cup quarter-final against a very much in-form West Ham side coming in quick succession afterwards.