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.