I don't get the Perk analysis either. It is pretty easy in my view.
Perk is a good rebounder, better than Krstic. Perk misses a lot of easy shots that get spoon fed to him under the basket, some of those misses leading to "easy" ORebs, but he is still a good rebounder, especially if he is asked to focus on this and not worry about screens or help defense (and KG isn't next to him gobbling up his share of rebounds).
Perk has been a factor in the improved defense in OK based on box scores and stats (I haven't seen a OK game so I can't assess it otherwise). This shouldn't be surprising, he is a good defensive player.
Statistically, the Celtics defense has certainly not been crippled by the loss of Perk but he is a good defensive player and his loss creates a need for other adjustments. I believe the Celtics defense is still really good, even though we lost a really good defensive player, because KG is an even better defensive player. KG in my opinion has a bigger impact on team defense than Perk.
I miss Perk, he was a good guy and I liked him on the team but I also am happy with what we got for in the trade him and I think the Celtics are going to do well in the playoffs.