The way Ruben Amorim & management have treated Casemiro was absolutely unfair

. Just for the record, he won everything with Real Madrid and that's why he went to Old Trafford to find some new challenges. But it seems like he was not given enough chances to actually when anything. Ruben Amorim has created Casemiro as a player who is just a backup plan for some other players. You guys know well, that he was definitely not a backup plan in the Spanish Soccer Giant.
I totally agree with what you've said, but I think this problem is more of like a managerial to player problem, and less of management to player problem. Now, when I say the problem is more of a managerial problem, it's very obvious. Players are considered lucky if they are opportuned to maintain their first team positions under two different managers, but in the case of casimero, things has not be so.
United's performance since the days of ETH has been very poor, making players like Casimero, and even CR7 look so unprefessional. And the other problem is less of a managerial problem because it already seem like they have issues with his high salary. He isn't meeting up to expectations, we understand, but United as a club isn't perfect either. The board just want to make examples out of others, rather than tackling the main problems.
The management of Manchester United was stupid to bring in such players and then treat them like they were not important to the squad. Sometimes it seems to me Ruben Amorim tried to make a young squad. But if you are going for a young squad, why are you bringing in Cristiano Ronaldo and Casemiro? Actually, the problem was the MU management did not know what they wanted.

I already addressed this.
Different managers with their own choice of players.
It takes luck for a player to retain his position in the first eleven of a different manager, but it seems in the case of casimero, the board might just have issues with his salary, as they've been cutting cost for some time now. Parting ways with players won't be enough, the board might just need new hands.