Mainoo is pushing to leave United in January, but fellow United academy graduate Giggs insists he should stay and fight for his place.
Speaking to The Sun via betting comparison site BetSelect, Giggs said: “Every player wants to play in the World Cup and be on the pitch for their club.
“But if he goes on loan somewhere, he’s only got three or four months to really hit the ground running. What club has he gone to?
“Is it a club that’s getting beaten every week, and it’s not doing him any favours? Or is it somewhere he’s going to be in and out of the team?”
Giggs added: “He needs to make sure that when he gets his chance, he does well, especially around this time when the games come thick and fast.
“I thought he did well the other night when he came on against Bournemouth. He just needs to take his chance, and anything can happen.
“If he suddenly takes these opportunities and he’s playing regularly until the end of the season for Man United, that will give him more chance (at the England Squad) than going anywhere else.”