The playmaker came off the bench just after the hour-mark and drilled home in the 67th minute to hand his side victory over the Eagles at Old Trafford.
Mata, 26, has started seven of United’s 11 Premier League games this season, but Van Gaal refused to guarantee the Spain international a spot in his starting line-up.
“Juan Mata has played a lot of matches,” said Van Gaal.
“I have given him the chance, I give other players the chance and at the end we shall see who plays more than the other. It is always like that in football.
“He didn’t change the formation or the style of play but he scored, and that’s always the most important thing.
“When you score, you win the game. Because of that we won. So I am happy with that.”
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