Real Madrid could always find a replacement for Casemiro. The guy is very old and not much more chance for him in Real Madrid in the next few years. I saw some unconfirmed news about Real Madrid wanting to find a Casemiro replacement, one of the candidates is Tottenham's midfielder, Pierre-Emile Højbjerg. This comes from a tier-2 news runner from England so not very sure but does carry some merit.
Aurélien Tchouameni is a player who has had a similar role at Real Madrid this season, so I don't think Real Madrid should immediately get a replacement for Casemiro should he be sold this season. Real Madrid paid a very high amount of money for Aurélien Tchouameni, so Real Madrid actually planned to sell Casemiro after signing the Frenchman.
Casemiro is also not too old to be a Real Madrid player for the next few seasons, but Real Madrid may need to balance his team's finances even though this season they don't have many signings. I don't think there is a problem if they sell Casemiro because in fact Aurélien Tchouameni has become a worthy successor.
It would be good for Real Madrid if they didn't sell Casemiro in this season. His performance is good enough and he is still playing like a young. He has capability to serve few more seasons. Real Madrid will definitely miss Casemiro in their middle order.