Well Casemiro is still a vibrant player, I won't be surprised if a club comes for him on loan or outrightly buy him despite his huge weekly wages, this Saudi guys aren't playing with you when they are coming for a player, so in the aspect of the salary I don't think any Club that really wants him would look at that, Manchester United just want to get rid of him, probably because he doesn't fall into the specification of what Amorim needs for his team, just the way he did for Rashford.
Vibrant players as how? Only a delusional team will want to spend a huge amount of money for him when he is not worth it currently. If they buy him for huge amount of money, what does he has to bring to the table? The short term hype he had in the first two months in Manchester United or the number 13th place Manchester United has slept on for many months now. He is going to get teams that will outrightly comes for him but don't expect money bag.
You are comparing Rashford to Casemiro, literally Casemiro has finished football the moment he left Real Madrid to join Manchester United but Rashford is yet to test the tip of his time in football. He was just caught between unfortunate coach and team that don't know how to make calls and don't even know what they want right now, they want to win but don't know what to use and how to use despite the changes and transition of coaches.