I must say, I can not help but laugh when someone uses cumulative stats to quantify Perk's performance, or, contributions. His overall numbers will never look great, or likely, even good.
There are a few reasons for this. First, yes, he just isn't as good as he was before the knee injury. Second, there are not a large number of teams with "big" centers. Many teams use PFs to masquerade as centers.
None of that matters once you are in the playoffs, going aganst guys like Howard, Marc Gasol, Tim Duncan, Bynum, etc. Perk is still highly capable of defending these type of centers very well. Being able to do so, in the playoffs, is worth its weight in gold. Not many guys can.
Perk earns that $9m paycheck in just a handful of games, the rest of the time, he's just a great "veteran leader".
I'd gladly trade Green for him right now. OKC wouldn't in a million years though.