Van Gaal's comments come after de Gea opted not to sign a new lucrative contract offered to him by the club.
With de Gea's current deal set to expire in 2016, there has already been a lot of speculation that he will join Real Madrid since Los Blancos have confirmed their interest in him.
"It takes a long time now," Van Gaal said during a news conference. "So … it can always happen. I am not the boss. I want him to stay. The player is the boss he can say yes or no. You will have to ask David de Gea not me. He has to sign. We have offered him a lot of money."
However, if van Gaal were to sell de Gea this year, he pointed out that United will have to start signing new players as early as possible.
"Of course it's an advantage but it's also dependable on the market, which players are free, which players want the clubs in the world to sell," he said. "It's not so easy and I don't want to buy every player."