I have no answer as to why he's suddenly playing bad defensively. At times he looks like the player from last season. Perhaps his struggles on offense are affecting his defense. It does happen with some players. But by no means am I ready to give up on him. By seasons end he should know what he needs to work on this coming off season, and show up better prepared at training camp.
Losing Darren Collison is a factor too. When Ben plays bad defensively it's usually because he's moving too fast, overreacting, trying to do too much. Having Collison out there calling out switches was benefiting everybody but Ben in particular. In his absence, Ray has very little experience running the team. Andre just got here. That puts more pressure on Ben and at this point in his career, increased pressure has been his achilles heel. Players that succeed right away in the NBA usually don't need a lot of specific direction. You could tell from watching Ben in college that he has a lot of talent, but it's going to take a lot of work to get him to reach his potential. I don't think that's a bad thing though, provided you understand what you're signing up for.