Casemiro is a very important player in maintaining the balance of the midfield. Manchester United have been very fortunate to have brought him to Old Trafford this season.
We can imagine if Manchester United didn't have Casemiro this season, they would definitely find it difficult to maintain their victory in various competitions including the European league. Casemiro's role at Manchester United will never be replaced, even if someone fills Casemiro's position when he is absent there may be another name like McTominay or Fred, but these two players are still not good enough to fill the defensive midfielder position.
Without Casemiro, Manchester United will lose balance in midfield. Casemiro absence is a loss for Manchester United, considering that Manchester United's winning percentage is quite high when Casemiro appears.
This has made it difficult for Manchester United to find a suitable replacement for him, but I believe Sabitzer, McTominay, or Fred are capable of replacing Casemiro role in midfield even if they don't completely cover the midfield properly. Erik Ten Hag knows what he is doing and will handle it well.
Casemiro is one of the best signing for Manchester United this season, his presence has played wonderful roles for Manchester United winning many games. I give credit Erik Ten Hag for bringing players like Casemiro knowing fully well he will be of good use to the club. Replacing Casemiro, Manchester United needs to look for someone that can take his space and play the same role if not Manchester United is unable to look for a good midfielder that can replace Casemiro in his absence it will seriously affect the team.