He just needs a rest. Casemiro is famous for being incredibly disciplined with his body and fitness. I don’t believe for a second his legs are gone or he’s finished. He’s notoriously slow to start a season, even at Madrid. He played a ton of football last two years. With Madrid all the way to the CL final, then a huge number of games for United as our key player, the winter World Cup, then full pre-season, then nearly every game and minute this season - where he’s been forced to cover more ground and do more than ever due to an initial change in system.
The guy just needs a break.
As the injury crisis eases, we should probably revert to the 4-2-3-1 of last season, giving him a proper partner, and rotate some games where he sits on the bench for Amrabat. Casemiro is a physical specimen, and plays a position that I think he can continue to dominate until he is 33 or 34, as long as he’s a true DM and mainly sitting and screening. He’s acted more of a box to box this season, hence him being our joint top scorer.
I have no concerns about this being a permanent decline. Just long term fatigue, coupled with increased demands of his role.