The Red Devils manager forced Shaw to train alone earlier this week after expressing his dissatisfaction with the teenager’s fitness.
I have got to get fitter to get up and down (the wing). It’s something I have got to work on
Shaw, who joined in a £27m deal last month, is expected to play in a wing-back role this season as Van Gaal switches to a 3-5-2, requiring the England international to patrol the left wing.
“Obviously I have seen that, and it’s something I totally agree with,” Shaw told talkSPORT after playing 45 minutes of United’s goalless draw with Inter Milan.
“To play in this formation you have to be really fit. I am fit but I am not at the high level he demands.
“I have got to get fitter to get up and down (the wing). It’s something I have got to work on.”
Shaw made 37 Premier League appearances at Southampton last season, producing more crosses and shots than any of his left-back rivals.
And the 18-year-old accepted that he underestimated the step up required after moving to the 20-time English champions.
“It was a surprise. I didn’t come into United thinking it would be much harder,” Shaw added.
“That’s something that I think I made a mistake on.
“I thought it was just going to be the same but it’s the biggest club in the world and there is world-class players in training.
“That is something that is going to push me on and it’s something I am looking forward to doing.”
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