The Spaniard was given his marching orders for the first time in his career for two avoidable bookings and although manager Louis van Gaal admitted one was stupid, he did indicate that referee Mike Dean could have taken into account his sublime disciplinary record prior to producing a second yellow and subsequent red.
"My opinion doesn't count," Van Gaal said when asked about Mata's red card.
"You give as a referee within five minutes two yellow cards to the same person and you know also who is Juan Mata.
"I asked him a rhetorical question: 'How many red cards (have you had in your career)?' None (he said).
"I had assumed that already so, yeah, as a player you have to know also when you have a yellow card that you have to take care of that."
The sending off ultimately cost United the match as Salomon Rondon beautifully controlled a cross from Sebastien Pocognoli before firing home in the 66th minute to give the Baggies all three points at The Hawthorns.