The Dutchman spent two seasons in charge of United, winning the FA Cup before being sacked by former chief exec Ed Woodward.
"They are a commercial club, not a football club," Van Gaal told Sky Sports.
"I have said that before and that’s always difficult. When the manager is not deciding which players have to come in, it is very difficult."
Asked if United chiefs or the manager should be in charge of transfers, Van Gaal continued: "I think that it is is the way that it should be (the manager should choose the signings).
"Because then you can fire a manager because he doesn’t give any results. But when other people buy the players, then you have a problem.
"You need to have the knowledge of the manager, the orientation of the manager, because he has to train them."