Perhaps going to Saudi Arabia will be an excellent choice for Casemiro and the teams. He may still be playing well now, but will he still be in good form next year? European football teams are prioritizing young players. They are fed up with injuries and the performance of older players. These players refused to take a pay cut and felt uncomfortable being placed on the bench. The perfect choice is still card bridges. But again, the older players are very experienced. They are always the pillars guiding young players. A team without experienced players will be a weak point.
Actually Casemiro have won many trophies with Real Madrid and he looks not really prioritize for many trophies with difference European teams, he has still on the top level but current condition still injury and can't speculate about his future keep stay with Manchester United or acceptable the interested from Saudi Arabia club. Have rumored with interested from PSG for signing Casemiro but depend on Manchester United offer will sell him or keep loan to PSG with his contract left two years for Manchester United.
I think too early for Casemiro if want play for Saudi Arabia club, his age still 31 years old as ideal age for top player in European but if have excited offering I don't think he will deny for joining with Saudi clubs.