I think the arrival of Casemiro will have a good impact on the center of Manchester United this season. However, for defense, it seems that Erik must immediately find another player to replace Maguire position because he has not been good enough in his defense in the last two matches. Casemiro arrival will also be a reunion between him and Ronaldo I think he will be the best passer for ronaldo.
Casemiro may make a small change in the midfield but it will not change Manchester United significantly.
The problem of Manchester United is almost in the whole team in all position as well as in how the coach manage the team.
Talking about passer, there is Bruno Fernandes and I'm sure he is better than Casemiro for this task.
Maguire is a captain, there's no way Ten Hag will remove him because he also trust Maguire.
This is one of Ten Haag's mistake to keep Maguire in the squad and take him as the captain make it even worst mistake.
He was great when he was in Leicester but he failed to show his best in Manchester United, even worst he made so many mistakes that made United lose many points in the last few seasons.
I have no idea why Ten Haag and Manchester United keep him in the squad while they can get a better defender.