So basically, Green's a player that only really plays well at Pierce's position. He's not a guy that you really want backing up KG. He's not a guy that you really want to plug into BBD's role next season. That limits his worth to the C's... and if he's not likely to get much more playing time next season as a result, makes it more likely he pushes to go elsewhere as a free agent.
Well, Pierce can also play the 2 against most matchups as well.
Basically, for this trade to be a success for THIS season, Green needs to be a very productive backup wing, and they need the centers they have to be healthy, and productive. They may use him at times against certain matchups at PF (many teams play SFs at PFs, which Green can certainly matchup against), but if Green can give them 15-20 minutes per game behind Pierce and Allen of high level basketball, then he will be a plus. And, if those changes are enough to get this team as far as they would have gone with Perkins, then the future prospects of the trade make it a good deal.
While Doc certainly was intrigued of using Green at the PF at times (and he really is not much different than Posey at that position...a little less defense, a little more offense, but both could be exploited by matchups), they got him to play on the wing.
The problem with the trade so far has been that neither of those requirements have worked out so far. The Centers have been injured (Shaq, JO) and inneffective (Krstic), and Green has been very mediocre.
All of that can change very quickly in the playoffs though, so, just like Doc said, lets wait and see how the season ends before deciding if it was a good trade.