Green does not defend as well as Bass. Here is a
great article that details some stats for Bass early in the season and it speaks to the intensity that he has had defensively this year.
As for the aggression, this year he had the chance to be 'the guy' offensively, the alpha dog. Yes he doesnt have the skillset to do that, but that doesnt stop guys like say Gay or Evan Turner from taking shots at a rate as if they are really 'the guy'. Thats what I would have liked to see from Green, not even changing his game and driving more, just taking more shots. He is one of our best offensive players and even if he is taking a lower efficiency shot for him its better than some of the other options we have on the team taking one of their most efficient shots.
Unless the Lakers are idiots they dont do that, they dont want Green clogging their cap. Furthermore, if you want to take on a $9M a year contract and allow another team to shed salary, there are more talented players than Green that you could do that with.
Also, in that Green for Nash scenario we get nothing but to shed Greens salary, I dont see what value we receive.