I think Dier is just not at the physical level required, he still needs time to work on his fitness. It's been almost a year that he's had a consistent, bizarre combination of illnesses and injuries, only at the start of the summer did he have an operation on a mysterious "issue" that was detected during medical tests on the first day of preseason. It was very strange how little the club said about that so perhaps it was something potentially serious.
If it isn't fitness issues keeping him out I find it strange how he went from Poch's most trusted player, starting virtually every game, to a complete non-entity in the space of half a season.