The Dutch coach says the Spain international must continue to impress if he wants to stay in his central role as the young Belgian-born attacker
The Spaniard has been used on the right wing for the most part this season, but he has started through the middle in United's past two games, helping Van Gaal's men to wins over Stoke City and Derby County.
Pereira has played just four Premier League games this term for the Red Devils and came on to play for the final few minutes against Stoke, replacing the ex-Chelsea man.
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Van Gaal is happy for Mata to continue in the role, but said the 27-year-old needs to keep impressing, otherwise he could be dropped for the 20-year-old Belgian-born attacker.
"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 previously he wasn't good enough there. That is the reason.
"It is not the coach or the manager, but when he is playing like he has been playing now, he shall play at No.10.
"We will have to see if he plays there on Sunday against Chelsea. Maybe I have another No.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.