Salomon Rondon struck in the 67th minute when he banged in his fourth goal in six league games to bag all three points.
United tried their best to stick it out and must have been hopeful that they would take something home until Rondon struck.
He was left totally unmarked in the box when a cross came in from the left and gave David de Gea no chance with a shot that crept just inside the post.
Quite what Mata was thinking of heaven only knows, but it was as daft a double booking as you are ever likely to see.
He complained bitterly to referee Mike Dean, but the Wirral official had absolutely no choice.
If anyone needed further confirmation that Mata had been guilty of some kind of mental block then you only had to look at United boss Van Gaal’s face as his mid- fielder went past him on the way to the tunnel.
It was one of total bewilderment and the disappointment of a man who had seen his side go a man down with over two-thirds of the game left.
Mata’s problem started in the 24th minute when he stupidly stuck a foot out to prevent former United man Darren Fletcher taking a free kick in no man’s land.
It wasn’t as if Fletcher could have quickly pinged a telling ball into the box. It was deep in the Albion half and out towards the touchline.
Once the referee had brandished that first yellow, Mata was always going to be treading on eggshells for the rest of the game.
Well, theoretically, anyway. Just 158 seconds later, the former Chelsea man tripped Fletcher and you just knew what was coming.