He played in Ruben's first game v Ipswich, partnered with Eriksen in midfield, and they both got the runaround.
I don't think that partnership has been seen since, but Casemiro seems to have been really put on the back-burner since then.
Maybe Amorim just went off him after that, doesn't see a future for him at the club and doesn't see any point in playing him?!
But at the moment, he's the best midfield partner for Ugarte. Mainoo's form has fallen off a cliff. He needs to be used more. That doesn't mean that he has do be a long term option for the team, but he has too much ability and experience to be twiddling his thumbs on the bench.
The unfortunate thing now is he may start against Newcastle but Ugarte is suspended, so he might end up partnering Eriksen again which would be a disaster!