Speaking to Sky Sports, Van Gaal said: "It is still a commercial club. It is not a football club. I have said that before - and that's always difficult.
"When [it is] not the manager deciding which player has to come, it's very difficult.
"Then you can fire the manager because he's not getting results. When other people buy the players, then you have a problem. You need to have the knowledge and orientation [of the manager] because he has to train them.
"[At Liverpool] Arne Slot is the man who says to his technical manager, you need to buy this player. Pay attention to it."