Shaw, 19, trained on his own at Washington Redskins' FedEx Field on Monday night as Van Gaal put the rest of the Manchester United squad through ball drills.
Van Gaal said: "I am always a trainer who sees the individuals and what each player needs.
"Luke needs to be fit and he's not very fit and can't perform how I want. He needs to be fit and to train individually.
"I can't judge why. I see what I see. I have spoken with him and we have made a programme with him.
"He has agreed. We will have to see how long it takes. I don't know.
"I have heard good messages from Tony Strudwick the fitness coach."
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