The Manchester United star and full-back Daryl Janmaat both reported stomach upsets on Tuesday, with Van Gaal leaving both to train separately as a precautionary measure.
And the Dutch coach says he will make a final decision on striker Van Persie in particular on Wednesday, just hours before the game at the Arena Corinthians in Sao Paulo.
“Van Persie had some intestinal problems so it was best he didn’t train today,” Van Gall told a press conference.
“I don’t know if he will be 100 per cent. I will have to take these decisions tomorrow. We have more than a day to go and Van Persie is a very important player to us.
“He is our captain, so in that capacity alone he is very important, and in principle my captains always play.”
The fitness of Van Persie and Janmaat are not the only reasons for concern for Van Gaal, with defender Ron Vlaar short of fitness and midfielder Nigel de Jong looking to make a shock early return from a thigh injury which had looked set to end his World Cup.
“De Jong tried out today and we will have to wait and see how it looks tomorrow,” Van Gaal admitted.
“If you want to return to the pitch so quickly there has to be a reaction. We will see how he reacts. Vlaar is the same. We’ll wait and see.”
Much has been said about Lionel Messi in the build-up to the crunch clash, with the Barcelona star expected to be the main man for Alejandro Sabella’s Argentina side.
But Van Gaal insists that while he will prepare his side to step Messi, the Albiceleste have many more players with which they could cause his side damage.
“Messi was once elected the best football player in the world, and that’s not for nothing,” he added. “At the highest stage he can always pull it off.
“In principle, he has always found it rather difficult to repeat that at national level and he wants to show he can do that. We need to stop him doing it.
“But we are playing against Argentina, we always refer to teams not to individuals. We showed the players some things on Argentina this morning, but I need to see how they will play and with which players before deciding exactly how we will play.”
The semi-final kicks off at 22:00CET on Wednesday, with the winners going on to play Brazil or Germany in Sunday’s final at the Maracana in Rio de Janeiro.