England international left-back Shaw suffered an injury against Yeovil Town in the FA Cup third-round tie away from home last weekend.
The youngster was withdrawn at half time and so was Rafael da Silva, but both players are fit to play against Southampton.
"They [Shaw and Rafael] can play," Van Gaal told MUTV. "They are both not injured. Rafael Da Silva was of course operated on [his cheekbone], and maybe we don't take any risk with him, but Shaw can also play.”
“Blind has already trained for more than one week with the group and Fellaini also trained with us already this week,” said the Dutchman.
“I have to tell you that we have only one injured player, Ashley Young, it's unbelievable! Another point is, of course, match rhythm, but everybody is healthy and can play except Ashley Young.”
“When you invite Victor Valdes to have his rehabilitation here at Manchester united, you do that for him, but also for yourself,” said the 63-year-old.
“I had the chance to see him and his behaviour in rehabilitation but also if his quality was still the same.
“He has confirmed both and that is very important for me and also for him because when I am satisfied then mostly the players are satisfied.”
Valdes is United’s first signing of the January transfer window, and there are reports that more players will be recruited.
However, Van Gaal is keeping his cards close to his chest and is not willing to divulge his plans in public.
“No, no, in this transfer window I do not discuss with you,” said the former Barcelona coach when asked if he will add to the squad this month. “You can ask in another way but I am always saying the same answer.”
When asked about the future of goalkeeper Anders Lindegaard, Van Gaal said: “I think that he [Valdes] fits more in the profile that I have of a goalkeeper, and that’s it.”
United will head into the game against Southampton placed third in the Premier League with 37 points from 20 matches.