Perk and JO really shouldn't be compared by the numbers. After seeing JO's impact defensively against the Knicks in both games, I no longer question whether JO has the capacity to replace what Perk gave the C's on the defensive end. In addition, Perk can score 20 points and no one could or should argue that he's JO's equal offensively.
Of course, the question is whether JO can play without a significant injury and whether he can up his minutes to 25-30 per game. Both of these are in doubt.
The reality is that if DA believed that JO and Shaq would be healthy, the risk is effectively mitigated. If they were healthy or Perk was hurt, or if Green was playing to his apparent potential, no one would be questioning the risk. But, 'risk' is an intersting word -- there is ALWAYS risk in any trade since anything can happen. And, of course, with the benefit of hindsight it's a lot easier to handicap a trade...