Casemiro is a defensive midfielder, not a playmaker. Just because Ten Hag is bad at using the tools at his disposal does not mean that the tools are bad, but rather that Ten Hag is a fool. In addition, the fouls he commits do not mean that he is to blame for them, for example recently when Onana made a bad pass and Casemiro had to make a foul to save the team from a goal, it was Onana's fault and not Casemiro.
Casemiro's arrival was very beneficial to Manchester united last season but this season he's been very sluggish and made many errors and barely mark an opposition player without getting warned or booked, however I feel his poor performance might be due to fatigue and he needs to be rested for a while to get him kicking again.
Maybe Casemiro works so hard in training (unlike Sancho) that Ten Hag is happy, but he simply doesn’t have the energy left for games?
I don’t know the reasons (I’m not inside the club) but I see the result and I know who is responsible for this result - the head coach. If your players degrade, it's your fault. Even Rashford has turned into a log this season. You can blame the players (which Ten Hag does all the time), but even if the players are to blame, it is his fault.