De Gea has been linked with a move to Real Madrid after rejecting a £200,000-a-week contract extension at United.
And Van Gaal did not sound confident when asked if the Spanish stopper would still be between the sticks at Old Trafford next term.
Asked if he could reassure United fans that De Gea would stay, he said: "No, I cannot.Â
You never know.
"But a transfer is a process, involving a player, a club and his present club. He has to choose.Â
"It's not easy to choose between two great clubs. He's Spanish and his girlfriend is Spanish, so we would be very happy if he stays.
"You never know."
De Gea came off during the game, making way for Victor Valdes to make his United debut.
And Van Gaal confirmed that his first choice keeper had been forced off with a hamstring injury.
He said: "I think David De Gea has a hamstring injury."
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