I'd envisage "right set-up" for McTominay being him playing as a second striker behind a no.9 who wants to come deep and vacate spaces for McTominay to box crash. Two central midfielders behind him who can defend and pass the football... Something we should not play because we have better players who occupy his best position.
That's essentially it; he's an attacking midfielder for a smaller club and he'll tally well enough for them in that role, but here, he shouldn't get that gig.
It's on the managers who put him in CM as by now, it's abundantly clear what he does or does not offer. I keep reading that he hides, but I always think if you don't actually know what you're doing or intrinsically have it about you, you'll always lag behind the play and look like an imposter/fraud or what have you. McTominay mostly looks like someone who is playing a reactive game in the most proactive area of the pitch, hence the delay on his actions and all the things that make a midfielder stand out to the positive or negative. If we then throw him into that same area of the pitch again and again and again, is he to blame or the managers doing it?
Mainoo is going to 'expose' him even further and that's not really McTominay's fault. Ones a natural, prodigious midfielder, the other has been plonked there and is winging it to no avail; we can't possibly pull the data from McTominay's games, particularly the amount of passes, touches and involvements he has in a typical game and then think, yep, roll him out to do that again, please.