The 19-year-old emerged as one of the Premier League's breakout stars last season, establishing himself at the heart of the Red Devils' midfield and earning a spot in England's Euro 2024 squad.
Mainoo's rapid rise from promising young talent to international starter saw him nominated for the 2024 Golden Boy award, with the youngster tipped to continue his strong form this term.
However, the Englishman has struggled for consistency, with injuries and positional changes limiting his impact.
Under new head coach Ruben Amorim, Mainoo has been deployed in various roles in a bid to find solutions to United's problems, who currently sit 13th in the Premier League table.
This included a recent stint as a striker in the Red Devils' last league game, a 2-0 defeat at home to Crystal Palace.
Read More on Man United
Liverpool killers Plymouth earn trip to Man City as clubs discover FA Cup fifth round fate
Man United's co-owners Ineos sued by New Zealand Rugby as bombshell statement is released
And club legend Scholes, who won 11 Premier League titles in 20 years at Old Trafford, has voiced his concerns over Mainoo.
Appearing on The Overlap Fan Debate, the ex-England international claimed United are unsure of how to get the best out of him, and compared his situation to that of Arsenal's Declan Rice.
He said: "I think Mainoo is lost.
"I've talked about Declan Rice being lost before - I think Mainoo is lost as well. I don't think they know what to do with him.
Most read in Football
I told Aston Villa to rip up my contract after what happened in training
Premier League stars make wholesome gesture after ex-Norwich academy captain dies aged 26
Sancho slammed for 'freedom' comment to Rashford, Alexander-Arnold and Saka injury updates
Liverpool killers Plymouth earn trip to Man City as clubs discover FA Cup fifth round fate
"I don't think he knows where to play on a football pitch because he's been played in one of the number 10 positions, he has been played in the two in the midfield, which I can see probably doesn't suit him, because I don't think the greatest athlete.
"He's a great footballer. I think the No 10 does suit, [but] the false nine, never in a million years."
Earlier on in the episode, Scholes voiced similar concerns over Rice, who has also faced a positional conundrum at Arsenal this season.
Traditionally a deep-lying centre midfielder, the England star has occasionally been played in more advanced roles this term by manager Mikel Arteta.
But Scholes questioned this, saying: "Declan Rice, I'm a little disappointed with him because I think he's been put in a place where we don't really know what he is anymore.
"Is he that holding midfield player? Is he the one, I've heard people like Gary Neville say he should be the one getting forward, scoring goals.
"I've never seen him that way. I think he's almost become a little bit stuck in not really knowing what he is and Arteta knowing what he is."
As well as his sharing his fears for Rice and Mainoo, Scholes also suggested United could face a relegation battle next season, if they do not strengthen their squad in the summer.
The legendary former midfielder questioned the Red Devils' overall quality, and admitted: "There are a few things that are concerning.
"There's a lot of stuff he [Amorim] needs to do, we all know that, especially in transfer windows, but I don't see a core of players.
'When you look at teams, you look right down the spine of a team. Liverpool: great goalkeeper, great centre half, forwards, midfielders, whatever.
"I don't think United have any of that in any position."
Read More on talkSPORT
Premier League stars make wholesome gesture after ex-Norwich academy captain dies aged 26
Super Bowl 60 logo revealed and conspiracy theorists think they know who'll make it
He continued: "That spine, that core is so important.
"If they don't get that right this summer, this might sound ridiculous but the form since this manager's been in, they could be looking at being in that relegation fight. I'm telling you, it's been so bad."