Well, you did call them average.
Peltier was a very good defender. Who skinned him? He was extremely limited but he could defend. If the ball went past him, the man didn't . Bennett is the other way but he's a decent left back. Cunningham again, is a decent left back. Maybe Harris is right about who's the better defender after Benkovic's "performance" tonight.
Just like at all levels, some players are better than others. Manga, Bamba and Morrison were miles ahead of most centre backs at this level.