Not Dier specifically.
I can think of a couple of occasions when Dunk was pulled out wide, covering for his fullback, and looked very shaky. I think one was against Fulham at the Amex when Mitrovic completely bullied him out wide and drove into the box.
The more I consider Potters preference for possession based football, the more I fear for Dunk and Duffy.
Both are excellent backs to the wall defenders, but neither excell playing a high line. Duffy prone to the odd lapse in concentration, allowing attackers to pick off a careless backpass...