The left-back joined the Premier League giants from Southampton earlier this summer and made his debut against LA Galaxy but the Dutchman claims he is not yet in peak condition
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."