Making judgment from preseason games is quite overblown. It's true for Green and for all the other players on the roster.
Green has been awful so far in the preseason but I wouldn't make much of it. I, like every Celtics fan, I guess, am waiting for this season to see if Jeff can finally put his inconcistency behind him and become a consistent scorer, day in and day out. He obviously has to prove it, I think we can all agree on that.
With that said, I think it's ridiculous to claim that there are two camps : one always making excuses for Jeff, and one claiming he sucks.
It seems obvious to me that he is indeed preserving himself and taking the preseason as a "warmup" session to get ready for the real NBA season. It's typical of veteran players and while Jeff is not a vet yet, his attitude clearly shows he is acting like one (like the simple fact that he is taking so many jumpers and not really looking to go to the paint or even have a real scorer mentality).
Obviously I could be wrong and I would clearly admit it if happens. But the last time I was claiming that a team was only waiting for the real games, and thus that we should not worry about them, I was talking about the French national team, that struggled in the first round of the Euro... and then went on to win the tournament, by trouncing Lituania in the Finals and beating their nemesis Spain in the previous game.

Of course, I'm not predicting an MVP season for Green but I'm pretty sure he will perform very well, as soon as the season starts.