Mainoo has been largely frozen out by United manager Ruben Amorim this season and is a target for Chelsea.
Given the situation, former United midfielders Butt - also the club's ex-academy director - and Scholes say Mainoo should now leave for Chelsea.
Scholes said on the The Good, The Bad & The Football podcast: “You would have to advise him to go. If he rang me and said, ‘I think Chelsea are in for me what do you think?’, I’d say ‘all day long’.
“He is a 20-year-old kid, he is going to have to leave. What will happen now is he’ll bring him into a game and he won’t be great, because he needs five, six, seven, eight games.
“He’ll throw him into the team and say ‘I told you so’. No one (in the hierarchy) gets the club. You think of the treatment of Kobbie Mainoo.
“He’s not playing, fair enough, sometimes as a manager you don’t fancy a player. When they aren’t playing well and what he has done in the game already as a young kid, winning goal in the FA Cup final, he’s played nearly every game at the Euros.
“I bet he takes the p*** out of Casemiro every day in training.”
Butt also said: “Kobbie has got to go. If you take your Man United head off, I am going ‘We are getting you out of that football club, you don’t need to speak up, we will sort it out’.
"For his football career he has to leave. He’s lost 18 months of development.
"I hope he goes abroad, it is embarrassing if he goes to Chelsea. They might go and get him, as they are putting together a very good young squad. They keep buying young players and the right good young players.
"You think he could quite easily end up at Chelsea next season.”