Casemiro is said to be a target by PSG and Al-Hilal now. You know he has been quite far away from his old performances recently. This is why Manchester United are said to be thinking of parting ways with him. His time with injury didn't end up well for him. He is still a quality player but he needs to improve his effort much more than this.
Casemiro would likely leave Manchester United, but i don't think it can be to PSG. I found it weird when United paid £70m for a player of his age, there is no doubting his quality, but when a player is already above 30, he should not cost as much as that, except it is a Saudi club that is doing the signing, lol.
PSG are looking for younger players, especially from France, that is the direction the club has decided to take, they no longer want to sign only big stars, but they want to nurture young talents, if Casemiro leaves United, Saudi Arabia would be a more likely destination that the French champions.