The Premier League side have been heavily linked with moves for midfielders Kevin Strootman and Arturo Vidal, as well as Borussia Dortmund defender Mats Hummels.
Van Gaal, in his first season in charge, said he still wanted to give his current squad a chance to stake their claims before potentially adding to the three players signed so far.
"I want to give the current players an honest chance. We are winning everything, maybe we don't need other players," the Dutchman told a news conference on Friday.
"Anyway, the transfer window still has a month left."
United face European champions Real Madrid in the International Champions Cup on Saturday in front of what is expected to be a crowd of 100,000-plus in Michigan.
Van Gaal, who ruled out defender Chris Smalling with a "minor problem", said the aim was to win, even at the pre-season friendly tournament.
"I'm anxious to win," the said.
"I'm a coach who wants to win and United are used to that too, so I think my players will show that attitude also."
Under a new 3-5-2 formation, United have claimed three pre-season victories.
Wayne Rooney, who has scored four times in three games, said he was happy to be learning a new system in the latter years of his career.
"It's been great," he said.
"Obviously the manager has come in and we're playing under a different system. It's been great even at my age to be able to learn a new system and try to get better with the team."
