The Spaniard has started centrally in United’s past two games, helping Van Gaal’s men to wins overStoke City and Derby County.
Van Gaal is happy for Mata to continue in the role, but said the 27-year-old needed to keep impressing.
“I have already played Mata in the same role. Maybe you remember that,” the Dutchman said.
“I’m not changing him from that role, he has changed himself because at that time he wasn’t good enough. That is the reason.
“It is not the trainer-coach or the manager, but when he is playing like he has played now he shall play at number 10.
“Maybe, we have to see if he plays there on Sunday. Maybe I have another number 10 who is better than Mata. Maybe Andreas Pereira is better.
“I don’t discuss that with you, I’ll discuss it with Juan Mata. Juan Mata knows, and maybe he is doing better now.”
United, who sit fifth in the Premier League, take on Chelsea on Sunday.