The United academy star has been one of their best prospects over the past 18 months.
However, Scholes fears that Mainoo is not strong enough to complete 90 minutes, as he was hauled off during their 2-2 draw with Liverpool at Anfield on Sunday.
“Mainoo with his legs has to be a worry,” Scholes told Premier League Productions.
“He doesn't finish a lot of games.”
Michael Owen added: “He looks heavy, but yeah, he tires a lot, doesn't he? And it's one thing he's an unbelievable player, but if there's one thing about Mainoo, it's getting around the pitch and his ability to last a full game at the high intensity that's needed.
“And then you look at the other alternatives. You've got (Christian) Eriksen that's getting on in years. Casemiro that's never been that mobile but getting on in years.
“So, I guess you're left with (Manuel) Ugarte and another. You're left with Ugarte.”