Academy players always get a longer free pass, whichever fan base we talk about
Thing is, you were so average last season that it wasn't hard for Mainoo to look good (nor Garnacho for that matter). I'd contend that he was good but not THAT good.
The weaknesses that many talk about were there last season too, but people didn't want to see them. Even when playing for England, I noticed that he had a tendency to stand tight alongside an opposition player, thus making it hard for one of the centre-halves to pass to him and get a move started, so it's not just a MUFC issue
But the main problem is that intensity is so important in premier league football nowadays (look at Bournemouth's play against Newcastle yesterday, for example) and I don't see any intensity in Mainoo's game at all. He could feasibly play in an Eddie Howe-style three-man midfield but in a two his lack of intensity is hugely problematic for his partner, whoever that is. I could see him being better suited to La Liga or Serie A, but Premier League? I don't see it
Part of the problem is his best position. another poster made a great point, I think, when they said he's 'positionally ambigious.' - is he a 6? Not at all yet, but could see potentially see him playing there as an older player. An 8? Most likely, but probably only in a midfield three for now. A 10? Some of his neat interplay suggests so, but his limited goal threat and lack of assists hardly point to him being a 10, let alone an elite one.
He's not as bad now as many are saying, just like he wasn't as good last season as many were making him out to be.
I still think he can be a good player, but surely you'd be mad to turn down any bid around 70-100m (though on current form his value is plummeting), especially with so many issues that need fixing in your squad - goalkeeper, striker, centre midfield and left wing-back