The German midfielder, who joined from Bayern Munich for around £15million recently, came on at half-time of United's 3-1 International Champions Cup win over MLS side San Jose Earthquakes in California.
Van Gaal then rung the changes at the break which decreased the output of the English giants with Schweinsteiger being pointed out for his underwhelming showing.
"He was also bad," van Gaal said.
"Players can play bad, certainly when you have only three days or six days training session in your legs and you have to play 45 minutes.
"Everybody can think it is easy to step in but it is not easy. I thought the first match (Schweinsteiger) played very well and now he played bad but that is also a process."