The Spain international picked up two yellow cards in the opening 26 minutes at The Hawthorns to leave Louis van Gaal’s side with 10 men.
I could have avoided them and I take responsibility
Mata picked up his first caution after he tried to block a West Brom free-kick before he fouled former United midfielder Darren Fletcher.
The Baggies scored their winner in the second half thanks to Salomon Rondon to inflict a first defeat in four games upon the Red Devils.
Writing in his blog on Monday morning, the World Cup winner was quick to apologise to United supporters for his sending off.
“After almost 500 games as a professional player, this Sunday I was sent off for the first time in my career,” Mata said.
“As you can imagine, it’s not easy for me to write these lines. The truth is this is a new and strange situation for me, not easy to assimilate, but we learn from everything.
“I know that, in football, we see things like these every week, but obviously what hurts more is that my team-mates had to fight with ten men for a long time.
“I have the feeling that both decisions were rigorous; that, in many occasions, we see more serious fouls that are not penalised in such a way, but at the same time I could have avoided them and I take responsibility.
“It also leaves me with a bad taste for the supporters who travelled to the stadium, and for those who were watching the game from home.”
The Red Devils are in sixth place in the Premier League table and six points behind fourth-placed Manchester City.
United make the trip to Anfield for the first leg of their Europa League last-16 tie against Liverpool on Thursday night.
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