I'm going to have to attribute OKC's success to having 2 top 15 players on their roster in Durant and Westbrook, and maybe the best 6th man in the league right now in Harden. OKC wanted to trade Green to free up playing time for Ibaka who is an absolute beast. If you want to look at a role player who is truly making a significant contribution it's Ibaka. Perk is just along for the ride.
Perk's PER is not only the worst (by far) of any starting center, it's also one of the worst in the league. Now given that, if you're still saying stats don't matter for a guy like Perk (I mean what the heck is that supposed to mean?), then the reality is there is absolutely nothing that would convince you Perk is a below average player.