The 18-year-old has started 12 games in all competitions for the Red Devils since breaking into the team at the end of February, scoring seven goals to burst onto the scene and cause some calls for him to be included in the England squad for Euro 2016.
Rashford scored the only goal of Saturday's match against Aston Villa but then came off with 15 minutes remaining, and Van Gaal has revealed that he has not been a full participant in training since.
"I have changed him after 70 minutes, I believe, and that is also to give him a little bit of rest," Van Gaal told reporters.
"He didn't train the day after and also the day after that he didn't train. I want to keep him fresh because he is in the momentum of scoring and that is very important for him but also for the team."
With captain Wayne Rooney fit again, Anthony Martial dropped to the bench at the weekend but should return to the starting lineup tomorrow if Rashford is rested with Saturday's FA Cup semi-final against Everton in mind.