Dier has been making mistakes frequently enough at right back for him to be dropped/rotated with Naughton. Solid PL player, but far from everyone's favourite player...
I see no reason to think he's ready to be a reliable, consistent performer at centre back at this point.
Fazio has gotten two PL starts since signing, after getting very little pre-season time with the team because of signing late. I think he should be given more time on the pitch. He was a fixture in the EL winning side last season and somewhere near the top of their list of leaders/captains.
Agreed, he needs more playing time before we can judge him.