Shaw joined United from fellow Premier League side Southampton after returning from England's dismal World Cup campaign in a deal reported to be worth up to 30 million pounds ($50.93 million), the highest fee for a teenage player.
On Monday, he trained away from his team mates during an open session at the Washington Redskins' FedEx Field on the club's North American pre-season tour.
"I am always a trainer who sees the individuals and what each player needs," Van Gaal told reporters when quizzed about why Shaw was training alone.
"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."
Van Gaal could not say how much work Shaw had to do to reach a satisfactory level.
"How long That we have to see, but that I don't know," he added.
"I have heard good messages from Strud (fitness coach Tony Strudwick) but we have to see."
Meanwhile, Wilfried Zaha, who has struggled since joining from Crystal Palace last year, is set to make his first appearance on the tour against Inter Milan on Tuesday.
Van Gaal said he was looking at utilising the winger as a striker as he continues to develop a 3-5-2 formation at the club.
"He will play in the second-half in the strikers' position.
"When we play this system he has to play as a striker, otherwise he cannot play. Ashley Young is a winger, but he is another type who can play wing-back and he did very well (in a 3-2 friendly win) against Roma.
"He can play left and right side, but we shall see. He can do the job. He believes me and he is now performing well."