Mitten also was saying up until recently that the mood at the squad and the support the manager has among everyone at the club has always been great and was nothing like in prior years post Sir Alex. So the "not responding to the manager" can mean loads of things. Either he is losing them, or just an observation that the tactics aren't working so "the players clearly aren't responding to what he's trying to implement", or focusing on the few players we know actually aren't with the manager like Sancho, or something else... It's a bit vague.
But ultimately, I don't really have that much sympathy for Ten Hag in situations like that. That will happen at big clubs where some players don't like you or don't agree with you. It's up to you to find a way to get it to work, get them performing or stop playing those players. It's not like we've seen drastic selection changes among this losing run. If players are questioning dumb tactical moves like "why do you keep playing Casemiro alone in there when he is clearly struggling there", it is a fair question. If they are asking "why is Garnacho not starting more", it's a fair question. If they are asking "why does Amrabat keep playing at left back when he is very far out of his depth there", it is a fair question. I get why he tried it, but he has to change it.