De Gea has improved year-on-year since moving to Old Trafford for £18.9 million in the summer of 2012, and a string of impressive performances this season have seen him earn a place on the shortlist for the PFA Players' Player of the Year award.
But the 24-year-old enters the final year of his contract this summer and with Real Madrid keen admirers, Van Gaal admits there is little he can do to keep De Gea if he wants to return to Spain.
“It takes a long time now,” Van Gaal said when asked about the status of De Gea's contract talks. “It can always happen, but I am not the boss.
“The player is the boss and he can say yes or no. I want him to stay, but you will have to ask David de Gea, not me. He has to sign, but we have offered him a lot of money.”
Van Gaal also believes United's attempts to strengthen their squad with big names this summer will be at the mercy of player power.
“At the end, the player has to decide,” the Dutchman added. “Not the manager and not the manager of the selling club. It’s also dependable on the market. The club is dependable of the market and the selling clubs and the buying clubs.
“It’s not so easy to finalise all the decisions you are making because the players are the boss, not the manager. Every time you read that I buy this player or that player, or have communicated with that player, it is not like that.”