Does playing in the SPL automatically make him a poor defender? No. Maybe he is thriving in the SPL because he is a genuine talent.
From what I've seen (which to be fair is relatively limited) he is big, strong, comfortable on the ball and reads the game pretty well.
Nevertheless, it is also reasonably fair to question his ability because of the standard of the league and the opposition he is facing.
The talent he is facing is obviously much lower, so arguably it must be easier for him. Has he been truly tested? In Europe he has seemed shaky!
So to be fair everyone's opinions are purely speculation until he actually signs for a Prem team and is actually tested by quality players and teams.
Personally, I don't think he'll be as poor as some may make out, but he isn't going to be a top defender. Middle of the table standard more likely.