Tell me why the Celtics traded Kendrick Perkins again?

I just don't get this trade and its clearly affecting their performance without a big, physical guy like him to give the Celts more muscle in the paint. And the guy's a proven winner, being a key piece to their championship a few years back. And he's only 26. Jeff Greene just doesn't fill this role and Perkins is now making a big impact in Oklahoma City doing the job for them. Why did Danny Ainge make this trade again? Is there another player he's targetting to fill Perkins' role or was this just one of the most bone headed GM decisions of the year? (The Celts have now lost 3 of 4 btw, somewhere, the Heat, Magic, Bulls, and Lakers must be raising a toast to Ainge).