The England international signed a long-term deal with the Red Devils after United agreed a £27m deal for the left-back with Southampton earlier this summer.
But 62-year-old Van Gaal, who has signed a three-year deal at United, was in unforgiving mood as he hit out at the 19-year-old’s lack of fitness.
“I am always a trainer who sees the individuals and what each player needs,” Van Gaal said.
“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. We will have to see how long it takes. I don’t know. I have heard good messages from Tony Strudwick the fitness coach.”
Here is a selection of the best photos from United’s training session.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news