I am not sure what his defensive stats look like, but I know that I cannot think of another player who can stay in front of him man while pressuring him better than Bradley. It's an amazing combination of quickness/ athleticism and a solid fundamental understanding of defense/ moving your feet.
Maybe I am wrong, maybe there is something I am missing, but in my opinion, Bradley is one of the best perimeter defenders in the league.
Now, in all fairness, he is slightly undersized, he is not a very good shooter, and he doesn't really create his own shot in half-court, so there are some major issues with his game, but his defense is simply not one of those issues.