De Gea was dropped by Van Gaal for United's 1-0 victory against Tottenham on the opening day of the season, with the Dutchman indicating that the Spain international was not mentally prepared to play.
Sergio Romero was handed his debut instead as De Gea watched from the stands alongside Victor Valdes and Anders Lindegaard but his manager says that a deal for the goalkeeper is yet to be agreed with the Spanish giants.
"I cannot say [when De Gea's future will be settled] because I'm not the most important part – that is the player and the club that wants to buy him," Van Gaal told BT Sport.
"He is our best player for three years chosen by the fans and we cannot let him go easily – we have to spend a lot of money for the players we want to buy."
THE GLORY OF ROMERO | Sergio Romero was thrown in against Tottenham in place of David de Gea
Van Gaal was pleased with the performances of new signings Romero, Memphis Depay and Matteo Darmian but is eyeing improvement from his summer recruits after a tentative victory at Old Trafford.
He added: "I told Memphis he doesn't have to play with too much eagerness and passion and that also counts for the whole team.
"I liked Darmian because he played a good match in possession and when the opponent had the ball. Sergio Romero kept a clean sheet and he did make some saves but he was not building up possession but that's logical. He has only been here one week and it's a process."