The Reds manager has declared a top four finish for his side this season.
Manchester United moved up the table from 10th to 6th position following their 1-0 triumph over Neil Warnock’s Crystal Palace.
Mata came off the bench to break the deadlock over the weekend and the Red Devils manager believes a top-four finish is back in their hands.
“I am happy with the three points,” Van Gaal said.
“We stay in the race to end the season in the places we’re aiming for, so that’s very important.”
Despite so much possession and less penetrative threat, the Dutchman was pleased with his team’s display.
“They came to defend and that is always very difficult to play as a team against,” he added.
“We had a lot of time on the ball, we created possibilities, not big chances, but possibilities.
“We could have scored in the first half but I said at half-time, ‘Okay, we played better than the matches before because we kept the pace before and the opponents will tire and there will come a moment we will score’.
“It was Juan Mata [who did] and after that goal I think we created more big chances because then they had to come forward, only we have to finish these chances because they were big chances.
“But I said several times when we are ahead in a game we have to kill the game and we did. So I’m happy, we have three points and we are two points behind fourth place so it is in our hands and that’s important.”