Manchester United played with 10 men for over an hour after Mata received two yellow cards – the first for blocking a free kick and the second for a trip on former team-mate Darren Fletcher.
“What can I say when you send off a player like Mata,” Van Gaal said. “I have asked him if it’s his first red card and he said yes.”
It was a surprise to see the usually cool, calm and collected Spaniard pick up two quick bookings.
Van Gaal was astonished by referee Mike Dean’s failure to take Mata’s mild-mannered character into question.
But the Dutchman hit out at Mata for his “stupid” first booking.