The Argentina international is yet to feature for his new club since joining from Sporting Lisbon as he waits for his official documentation to be processed.
Rojo, 24, has been at Argentinian Embassy in Madrid this week as he bids to secure his right to work within the EU.
“I cannot change the law of Argentina,” said Van Gaal.
“I am Manchester United manager, which is the biggest club in the world, but I cannot change the law. It’s only a matter of time.”
Fellow new signing Angel di Maria could feature for the Red Devils at Turf Moor, however, because he has an Italian passport thanks to his grandparents.
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