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.
Coach is always responsible for the performance of his players and the result of his team in each game regardless of injuries, sickness and red cards. He has to maintain his players fits all the time and has enough good substitutes to replace and rotate them when necessary.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.
Manchester United isn't okay this season and Ten Hag is under pressure to get the team back to its performance.