I don't see any reason why Casemiro would be interested in such a move - he feels great at Real Madrid, which is now a top club, while United have already dropped out of this status. I believe that United have such an interest, but in order to even begin discussion of this transition, United must offer an absolutely incredible amount of money.
Many times things happened without any reason as same happening here right now Manchester United officials are in Madrid for few talks, and we can hear something in next few hours or days which could be officially as they are trying their best for having some quality midfielders after losing their links with De Jong and Rabiot now they are with some other options and having good offers for players and clubs but here Casemiro is also not interested to join Manchester United, and they are also in talk with Atlético's Matheus Cunha which is possibly happened.
After these now we have another rumor again in progress Ronaldo is linked with Atlético Madrid, and now they are also offering Antoine Griezmann in exchange for this transfer, hopefully things will be positive for them as right now they are at bottom and want to do something positive before any big disaster.
Today I saw the news that United are ready to double Casemiro's salary (14 million instead of 7 million) and as I understand it, this is an after-tax amount. Maybe he will be interested in this, we must wait and see how events develop further. The same goes for Ronaldo, I see ongoing rumors about Chelsea and some other clubs, but most likely 90% of this information is a fiction of journalists, so now it's easier to wait for the end of the transfer season and see what really happens.