I have a feeling that Casemiro stills try to find out what position he is playing, what we want from him and what areas to cover.
Casemiro has not "lost his legs", he never had any much better then what see right now - the problem is that Casemiro is not player for 4-2-3-1 but instead ideal in 4-3-3 where he had 2 of the best CMs in the world in Kroos and Modric taking turns in who is the one dropping to take the ball from Casemiro, carry it forward or pass the ball to the final third and the other one would always make a run to cover the position were the dropping one started from just to disorient the defense and stretch out the midfield so the one that dropped gets a more open field to either carry the ball 20-30m or just slide a pass through the middle.
Casemiro a is decent passer, but mainly for functional football and not for Tika Taka football. He is not a playmaker, he is not box-to-box, he is not a CM - he is (and should be played like one if we are keeping one) low sitting CDM who is there to be the ball magnet winning duels both on the ground and in the air, make the area in front of our CBs and behind our 2 CMs in a 4-3-3 formation. He should also have the feeling when to slow down the pace of the game and when to play it faster, because the CDM is often the start of counter attacks.
But we dont have the players for a 4-3-3 and Casemiro is not longer the player we should be adapting or shaping our team after, was more an input in how I felt when he arrived and how I still see it.
His period was last year when he was playing as CM with his partner being the one often staying to cover up when he went on a goal streak haha.
Another big name that we had such high expectations that will not really give us what we expected and pay for.