The surprise result at Old Trafford extended United's losing streak to three, with previous defeats coming at Chelsea and Everton.
The Red Devils are now just four points clear of fifth-placed Liverpool, with Tottenham able to pile on the pressure tomorrow afternoon when they play their game in hand against Manchester City.
We know that Liverpool and more teams are coming from behind
Juan Mata
And Mata insists his side must learn from their poor run in order to keep the likes of Liverpool and Spurs at bay.
"It's a frustrating afternoon for us. If you don't score, it's impossible to win.
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Juan Mata urges Manchester United team-mates to learn from West Brom defeat
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"We know some teams will do this and we have to find a way to beat them. We need to learn from this and keep going.
"We know that Liverpool and more teams are coming from behind so we need more points as soon as possible."