Derozan isn't a bad defender. It's Triano's mandate that the wings deny everything and anything. Gerald Greene could be sitting 2 feet outside the three point line and Derozan is going to be playing in the passing lane up the line because that's what Triano decided over three months ago. It's completely amateurish and predictable. You can't just have a system and say this is our system. You have to play your opponent.
Derozan is an average defender guarding the hardest defensive position in the league on a horrible defensive team with a simple-minded defensive coach. It's only his situation that is keeping him from excelling on defense.
Nonetheless, the man still finds opportunities to create turnovers on a consistent basis.