The highly-talented teen midfielder only made his first start for United last Sunday against Everton in the English Premier League fixture.
The 18-year-old has been superb for the Red Devils in their last two games.
Mainoo made his first league start for Erik ten Hag’s side in their 3-0 win against Everton.
The English midfielder was surprisingly on the bench for United’s thrilling 3-3 draw against Galatasaray on Wednesday night but held his own after coming on.
And ex-United winger Sharpe believes the academy graduate should be ahead of Casemiro and Amrabat in the pecking order for Saturday’s Premier League clash against Newcastle.
He told 888sport after the Champions League game: “He looks like a natural in the team. He slots in and doesn’t panic in situations.
“There were a couple of players around him last night, but he kept the ball and displayed some nice touches, he just never gets flustered.
“He always looks composed and knows what’s around him – he comes across as an experienced first-teamer, but he’s only been in the team for two minutes.
“I’ve got high expectations for him – hopefully the manager and the club can protect him and play him at the right times.
“When you’re bringing off your new signing, Sofyan Amrabat, who you’ve paid millions for, to replace him with a young lad, it says a lot about his ability.
“You’ve also got Casemiro not playing at the moment, and I think Mainoo can keep each of them out of the team – I reckon he’s better than both of them.”
The Stockport-born star joined United’s academy at nine years old and after excelling through the ranks he signed his senior contract last year.
An ankle injury after a fine pre-season ruled him out of the start of the campaign.
With the midfielder now fit again Ten Hag will decide whether he starts again in an important game against Eddie Howe’s side on Saturday night.
United are one point above the Magpies in the table.