I hate to say it but am I ever wrong about a new signing. This guy is finished, Madrid sold us a dud. No wonder Ten Hag doesn't start him. He doesn't suit the type of football Ten Hag wants to play. He's a destroyer type DM and what we need is a possession based DM like Carrick/Busquets. It's another 60m down the drain and he'll probably just be a last 10 minutes off the bench player which is what he should be at most. Early days I know but I don't think he's going to be a good player for us, he's another player that lacks mobility like Maguire and they struggle in the premier league. Clearly he's not as immobile as Maguire, but he's not the quickest and not the youngest which is going to cause him problems.
If you think Casemiro is finished is because you thought he was a type of player he never was.
Casemiro will probably end up having a good stint at United but his level will be directly related to how good or bad the performance of the whole team is.
Not being a creative DM archetype he can be a great sidekick in midfield to players like Eriksen, Bruno or Fred. Before that ten Hag will need to manage how to make things work between CB's and midfield, he can put Antony on his first game because he mostly has to stretch the pitch, dribble and track back on defense, things that ten Hag knows he's more than capable to do and knows how to do in his system.
But for Casemiro to start playing at a good level, they have to go over a lot of things at the build and transition parts of the game, he can't just chuck him in and expect a 10/10 performance because he's not that kind of player, once Casemiro knows all the flaws of his teammates, his area of action and all his options when he has the ball he'll be ready to start getting important minutes.
And it's also happening with Ronaldo, who is moving to places of the pitch where he probably isn't supposed to be, at least no other player that has played more minutes than him comes deep into midfield to help the midfielders, because ten Hag wants his midfielders to be able to sort their problems out while the forwards stretch the pitch giving players like Bruno or Eriksen space to control the ball and threat backlines.
Some players find it easier to naturally assimilate the concepts of the current coach, in Casemiro's situation he has to see the big picture of how he wants/needs to play before a coach puts him in the field to be thrown at the wolves.