Shaw, who represented England at the recent World Cup finals in Brazil, completed a high-profile move to United from Southampton last month.
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Shaw trained separately from his team-mates at Washington's FedEx Field, ahead of United's International Champions Cup fixture against Inter, before sitting out parts of Monday's session.
Asked to explain why, Van Gaal said: "He is OK, but I am always a trainer-coach who sees individuals and what they need. He needs to be fit and is not very fit or fit enough to do what I want. He needs to train individually until he is fit."
Van Gaal admits he is unsure how long it will take for Shaw to reach a level of fitness he finds acceptable, but says he has received positive messages from the club's fitness team.
"I cannot judge how long that will take, but I see what I see," added Van Gaal.
"I have spoken with him and we have made a programme for him. He agreed with me.
"How long [until he is fully fit]? That we have to see, but that I don't know.
"I have heard good messages from Strud [United fitness coach Tony Strudwick], but we have to see."