Wayne Rooney, Anthony Martial and Marouane Fellaini are available for Manchester United's Champions League match against PSV Eindhoven at Old Trafford but all three have been told they must prove their fitness if they are to start on Wednesday.
“It is not automatic (that Rooney starts),” Van Gaal said. “It is always a decision of the manager.
“Fellaini, Rooney and Martial have to show their fitness and then I have to compare which are the best players to fit in our game plan. That is the same procedure every match.”
Ander Herrera, Phil Jones, Michael Carrick and Antonio Valencia are out through injury, but Bastian Schweinsteiger could feature despite missing the open part of Tuesday's training session.
Meanwhile, Luke Shaw has been given permission by manager Louis van Gaal to stay away from Old Trafford in case he feels it will reopen the psychological wounds of his leg break in Eindhoven.
Shaw’s season was brought to an abrupt and painful halt two months ago when he broke his right leg in two places following a tackle by Hector Moreno.
The 20-year-old was enjoying his best run of form at United, but the challenge from behind by the PSV defender left his leg bent at a 45-degree angle.
Shaw had an operation in Eindhoven before returning to England where he has since undergone another procedure.
Injured players are often seen watching United games from the stands.
But mindful that seeing United play against PSV could stoke some bad memories, Van Gaal has given Shaw a few days off from his rehabilitation which, the Dutchman admits, is proving to be a frustrating process for the England international.
“I think he is dealing with his injury very good, but now he is a little bit fed up with the daily routine so I give him four days off – also because of PSV,” the United manager said.
“Firstly, he was fed up with the daily routine. And secondly, I thought why he is fed up, maybe it is because of the PSV match and every player has to watch the game from the stadium to support their colleagues.
“I don’t think he will be here, but I don’t know. Maybe he surprises me.”
Van Gaal said after the game at the Philips Stadion that Shaw could be back by March.
That came as a boost to England manager Roy Hodgson, who has not given up hope of taking the left back to Euro 2016.
But Van Gaal is now preaching cautious words about the defender’s return date.
“I cannot answer that,” Van Gaal said when asked if Shaw would play again this season.
“You have to be very cautious. With an injury like that it can be going very well but one week it can be going very bad and then he is months behind schedule.
“He has been given four days off. On Friday he has a scan and we will know more about how the injury is going.”
Van Gaal was not happy with Moreno after the match in his homeland.
The former Bayern Munich boss declared the Mexican, who played under him at AZ, had made a “very bad tackle” and United officials were unhappy the defender was named man of the match after the 2-1 win in which he scored.
But Van Gaal has since revised his opinion and his players will not target Moreno in Wednesday’s match at Old Trafford.
“No, my players are not doing that,” he said. “I am sure of that. It was not his purpose (to injure Shaw).”
Having dropped just two points since the defeat to PSV in their opening group match, United will qualify for the last 16 if they beat PSV at Old Trafford.
Provisional squad: De Gea, Romero, Johnstone, Rojo, Tuanzebe, Smalling, Darmian, McNair, Mata, Blind, Young, Schneiderlin, Powell, Fellaini, Schweinsteiger, Lingard, Depay, Pereira, Rooney, Wilson, Martial.
