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.
There's alot of blunders in Manchester united squad, from the goal keeper, André Onana to the frontline, this case can not be sorted out currently, it will definitely take time. Erik Ten Hag practice favoritism in his squad, choosing the players he likes and dumps the ones that do meet up but because they fall out of favour under him, he dropped them, this case has happen to Cristiano Ronaldo and now is Jadon Sancho. Erik Ten Hag is the man killing Manchester United slowly with his bad character towards his players. Casemiro is aged and have achieved major titles during his time in Real Madrid, playing for United is more like having extra time to enjoy football with the English side, not longer competent enough.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.
I think Man Utd needs a whole reorganisation of the team. Starting from the coach then to all of the players. Onana at goal is critical,Maguire im not even gonna comment. Midfield looks okay in attack they need better players. So many players getting removed from the squad and i think if they dont sack Ten Hag its gonna get critical