The left back joined the Premier League giant from Southampton and made his debut against the LA Galaxy, but the manager 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.
However, it would appear he has work to do if he is to earn a place in Van Gaal's lineup for the Premier League opener against Swansea City at Old Trafford on Aug. 16.
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Shaw trained separately from his teammates 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 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 says he is unsure how long it will take for Shaw to reach a level of fitness he finds acceptable, but added 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," Van Gaal said. "I have spoken with him and we have made a program for him. He agreed with me."