Firstly, Casemiro is suspended about half a dozen games a season, secondly, his legs are starting to go, and thirdly, he will gradually replace him as first choice number 6.
Also, in some games, particularly away from home, I expect us to revert to the more robust 4231, with Casemiro and Rice as the double pivot.
Having quality players gives a manager options. When we have players on the bench who the manager does not trust, we can't afford to be flexible, so picking three from Casemiro, Rice, Mount, Eriksen, Bruno and Mainoo suddenly looks much better than finding three from Casemiro, Eriksen, Bruno, Fred, Mctominay and VDB, where the first three pick themselves.