Check out his stats over a period of time.
I wouldn't class myself as anti CT and I've little doubt he's our best current option defensively at left fullback. But your wording suggests his stats aren't what they were and in the here and now it doesn't really matter what they were three or four years ago.
If you could provide a reference source (please) for your stats that would be greatly appreciated. And what metric are you using to define 'best one on one left back in the Championship'?
Does he top the charts defensively in tackles, blocks, interceptions, crosses prevented and successful aerial challenges or score a steady average? Does he top the charts in progressive plays, crosses, successful crosses goals and assists for his position?
Not having a pop here as I've spent zero seconds cross checking his stats across the other 22 first choice left backs in the Championship. Do we then consider how his piers at other clubs would fare in our set up or he in theirs'? Genuinely intrigued.