United officially unveiled Schweinsteiger, Darmian and former Southampton midfielder Morgan Schneiderlin as their big-name trio of summer signings in Seattle on Wednesday, ahead of the club's opening pre-season fixture against Club America.
Van Gaal told reporters he was confident Schneiderlin's Premier League experience would hold him in good stead for his move to Old Trafford, but he was curious to see how the other two adapted.
"Morgan can cope with the Premier League," van Gaal said.
"That is the difference. There is a big difference between all the leagues and I have trained in them all, so I can judge that.
"But I am fully confident that Bastian and Matteo can cope with the rhythm of the Premier League."