The fact that a lot of other players have dramatically improved with ETH even since August (McTominay, Fred, Dalot, Shaw, Rashford) has made me cautiously optimistic about Maguire.
He looks a bit slimmer from recent training photos, which might improve his pace and agility. The players around him improving will also be helpful, with De Gea sweeping a bit more, Casemiro actually defending the space in front of the back line, and Martinez being an excellent ball mover.
I think he developed some bad habits from having opposing forwards get free runs at him several times a match. On a lot of those he had little choice but to lunge in for a tackle, so that started to be his automatic reaction to everything. Let's see how it goes after getting a significant break from matches and a few months of the new manager and coaches giving him instruction.