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.
This was a similar thing I was telling a United fan when they lost against Galatasaray, Erik Ten Hag is not a very bad coach but I think he's poor in his decision making this season and it's affecting the team also causing them to lose some games when they should be winning comfortably, well all know he's lacking options in his squad depth due to the injury crisis in the team currently but he should be able to manage the team he's got and put the rightful players at the rightful position.
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.