The Spanish goalkeeper made an instinctive save from a Jose Fonte header to ensure Manchester United maintained a 2-1 lead with half an hour of the match remaining.
Van Gaal praised the save and said it showed why United were so keen to not let the player leave for Real Madrid in the summer transfer window.
“He is the goalkeeper to do that. Also Sergio Romero has saved us in games, now David de Gea did that with a remarkable save from a header from Fonte.
“It was fantastic, therefore we are very happy he shall stay with us. That’s why we were very difficult in the transfer period for Real Madrid. It is not for nothing.”
Manchester United sit just two points behind Manchester City in second place in the Premier League, but Van Gaal insists it is too early to talk about his side winning the title.