I feel that I do have to defend Dier a bit. He gets compared to players like John Stones and Gary Cahill -- both players who've played in one position for just about their entire (albeit short) PL careers. I'm not even sure how many different formations they've played.
Dier's played (and presumably trained) at RB, DM and now back to CB again -- and even then, there's a vast difference between playing CB with a solid partner in a back 4 and then playing as (r)CB and having to cover one of the league's most attacking WBs. The positions he's got to take up, the angles he's got to cover, they're all different. Add to that the fact that he's still young and now playing in positions where every mistake leads to a scoring chance. And that he's got Toby next to him -- a blessing in that, well, he's an absolute Rolls-Royce of a defender, but a curse in that anyone* is going to look worse by comparison.
* Except Ledley of course. May his knees rest in peace.
Both Stones and Cahill were highlighted as a bit shaky after their big-money moves to clubs in the spotlight. They've settled down now (well, Cahill has) and are looking good. Hopefully that'll be Dier in a couple of seasons. Wherever he ends up playing.
If Dier settles into CB, rCB or DM, be it with us or another club (and I really hope not), I'm confident that he'll be viewed as one of the best in the Prem eventually. And a real leader.
If Dier keeps chopping and changing, he will still be (and already is, to be honest) a very good utility player -- someone Poch can trust to do a job anywhere in our two thirds of the pitch.