That seems like an easy out breh, his rebounding rate is dropping year by year and the Clippers haven't improved by a large margin as a team in regards to rebounding. The rate at which he's rebounding dropped off significantly compared to last year, he was playing with Jordan then as well.
Just seems like he's not giving max effort on the boards, for comparison, Kevin Love's rebound rate hasn't dropped despite playing with a much better rebounder in Pekovic (Over Darko).
Easy way out? Naw. As I said it's not about the Clippers improving in that area, it's that the minutes/roles are more distributed throughout the frontcourt. Plus his role has changed since his rookie season on the defensive end as well.
Clippers forward Blake Griffin disagreed with Del Negro.
"It depends on our defensive strategy and our defensive principles for that game," Griffin said. "We switch them every single game. I don't see that, no, but I'm biased." [...]
"Our main focus of practice and our theme of practice has been defensively making sure we're executing our game plan," Griffin said. "Because like I said, we switch it up every game, depending on who we're playing and who has the ball and who's a threat for them." [...]
"We want our identity to be a defensive team," Griffin said. "We don't want to have to make adjustments for every single team and switch things up but to make teams adjust to us and the way we play."
He's playing further away from the basket on offense this season =
2011/2012 - 4.3 offensive rebounds per 48 mins
2012/2013 - 3.3 offensive rebounds per 48 mins
Made a more concerted effort to play defense and not just look for rebounds (staying longer away from the basket on perimeter defense) =
2011/2012 - 10.0 defensive rebounds per 48 mins
2012/2013 - 8.8 offensive rebounds per 48 mins
Love's rebounding rate probably won't drop much regardless of team mates, that's why he's an exception to the rule.