Pereira has featured heavily for the Red Devils this season, making his 35th appearance in all competitions in Thursday night's Europa League draw with Club Brugge.
"He's had the second most appearances for Manchester United this season," Hargreaves said on BT Sport.
"Right now the load is being put on these young guys and [Marcus] Rashford and [Anthony] Martial and Mason Greenwood and Pereira.
"And really, they're going to be really good squad players for Manchester United but if Manchester United want to win the Champions League and win the Premier League, I think Pereira is going to be a good squad player.
"He's played multiple positions this season. He spoke about goals there [in his interview] but I think he's only got one [this season].
"At times he's played as a 10, at times he's played wide, today he's almost playing as a sitter. I think it's difficult for any young player coming into a team that's struggling playing multiple positions.
"So I think he's done well, he's had a breakthrough season but to expect him to get 15 goals, that's unrealistic right now."