I'd actually say Dier has never been a great CB, and certainly isn't suited to being the senior (organizer) CB in the pair.
His best games were in DM (higher up the pitch to your point) where he dropped back into the center of a three when the FBs went forward.
His decision that he doesn't want to play DM has hindered his value for the team, see first point.
Added to the challenge, our pairing has had some balance issues with gaps in aerial ability, speed and the left side issue that has not helped Dier or Toby.
I think he and Toby was the best of a bad setup last season, but I do expect he will be the last to leave (but from all conversations he is up for sale) simply because Toby is gone, lots of interest in Sanchez and CCV.