Literally every player that gets transferred to United eventually starts to underperform and fall off a cliff compared to their past performances and the conversation always ends up being how United has overpaid for said player or how they weren’t cut out for premier league or how they’re too old now but very rarely do people bring up how United might just be a graveyard for talent and the systemic issues at the club make the players worse instead of the other way around. It happened with Maguire, Sancho and Anthony. Now Casemiro and Varane are in talks of being possibly sold and the conversation around that is again revolving around how Real scammed United and that they’re falling off because of their ages (Varane is literally still 30 and Casemiro is 31). I’m willing to bet that if Doku had moved to United instead of City, he would’ve also had an incredibly difficult time and people would be laughing at United saying how they got rinsed paying 70 million for a talent that performed well in Ligue 1 for a single season. I think we should really stop judging players too harshly based on their performances under this United side.
I think it was more a case of Casemiro wanting a new challenge and the club respecting his wishes. Both Modric and Kroos are significantly older than him and are still playing, I’m sure he could’ve continued for a few more years as well if he had wanted to.