Very simple. Because he is very poor in one to one situations out wide he does not suit playing left of 3 CBs. Looks like now we have a good LB we have reverted to 4 at back. Since Dunk injured at last Burn has had chance to play in his proper position.
Will be interesting to see what Potter does when Dunk is fit. Retain Burn and go back to 3 CBs or continue with 2 FBs and leave Burn out. Tough on Burn but feel it will be latter. Of course also option of leaving Dunk out.
Suddenly Duffy well down pecking order
Pretty much how I see it. He is poor one on one simply because he lacks pace, which is hardly his fault. Other attributes are outstanding though - his positional sense is constantly improving and he is decent in the air and a good tackler. His passing has also improved massively.
I think he is a player who will look back at the end of his career and appreciate just how good he became under Potter's stewardship.