It's not Championship level, is the point, and should be more than capable of standing up to Coventry City. Maybe if you've got a dodgy £40m right back, play him at least in his natural position and have the player who has actually played a fair bit on the left, on the left??
Are we saying now that Casemiro is truly so done that we're scared to play him in central midfield against Coventry?
I think these players are bad and should be replaced if we have ambitions of challenging at the top, but acting as if we couldn't expect them to hold up against that opposition is an absurdity.
As a CB, Casemiro is probably championship level. As a LB, AWB too. Maguire has excellent qualities, but some serious downsides too. Championship might be a slight exaggeration, but it's really not far off. Pair that with the midfield setup at that stage of the game, 2 of the worst wide players we've ever had(on current form) and an inexperienced CF, yeah that team was pretty poor.
Yet it did still take 2 pretty big slices of luck to get them back in the game.
The manager took a gamble in his choice of subs, for sure. We aren't strong enough as a squad right now to be able to rest important players. But he's been trying to rest Mainoo and Garnacho whenever possible for months. That's to his credit imo.
As for playing Scott in a back four... that would also have been a gamble.
edit: Also, the price is the United price. It's not what those players are really worth if any other team is buying them, so is kind of irrelevant.