Casemiro is not coming into a well defined system or team though. Even if he comes we still don't know what the best 11 is.
Also, he's a good player but he's playing alongside Modric and Kroos two absolutely outstanding players who can make good players look great. We don't have that.
Now Cas is a good player but we need more than him and he needs more than what we currently have. Bruno with his god-awful erratic passing, Eriksen isn't as energetic as he once was, Fred is so-so and the less said about McTom the better.
If we sign Casemiro good, though I obviously have my doubts, but you need to put him alongside similar class.