Casemiro is getting and that's what we know about him but this player is still talented and he had a good performance in Manchester United when he was playing for this team as the first squad. Also, I remember in any game they didn't use Casemiro Mancshter Uniyed has problems and that's a good reason to say Casemiro is an effective player for his team. There are definitely many teams that want to hire Casemiro like Saudis and PSG but I think Manchester United should keep Casemiro for a longer time even if he is getting old.
Talking about Casemiro he's going through a difficult time at Manchester United because this season he was got injured it make his performance was decline and until now he still attempt to recover from his injured but based on the news i have read recently PSG has been refused the rumour about they were linked to Casemiro this because Luis Enrique has been said PSG currently has several players who play the same position as Casemiro so PSG have no plan to bought new midfielder this January even Luis Enrique has been thinking to sell some of PSG midfielder players after winter transfer opened besides that Ten Hag also doesn't want Casemiro to leave this because Ten Hag still need him because admitted or not for holding midfielder so far only Casemiro who able to performing well everytime he was played so this rumour most likely will not going to happendIt is true as you said that since Casemiro was brought from Real Madrid to Manchester United, the defensive midfielder's career has started to fade due to frequent injuries and at Manchester United he no longer has the winning mentality like when he was still at Real Madrid, even though the midfielder has good potential but unfortunately at Manchester United it doesn't suit his style of play and the Manchester United squad is very different from the Real Madrid squad which has almost equal strength.
Yes, maybe if he was at PSG it would further increase his confidence and be able to have a winning mentality which would make him enthusiastic to continue his sharp career, and do his best but maybe if he was at PSG he would only have the mentality of being a domestic league champion and not like Real Madrid or other league clubs another premier that has similar potential.
However, unfortunately the rumors about this player going to PSG are not completely certain because this is still just a rumor and we will see the truth during the January transfer window, whether PSG intends to bring him to the club, but I doubt it because Casemiro age is getting older if it's a long-term investment, it doesn't seem good, especially Midfielder have a track record of frequently suffering injuries.