Lamb was pretty good on D when he was actually in the rotation and getting valuable minutes, I have to think a big part of the bad D is that he's seriously discouraged with his situation. Brooks benched him for three months for no reason.
That and he has zero experience in the playoffs. I get Brooks is being forced into rotations a bit without Ibaka healthy but that's what happens when you jerk young players time around they end up a step behind in the playoffs. It doesn't help the Spurs are just balling.