The Evan Turner signing makes a lot more sense if you view him as a cheap short-term replacement for Green leaving.
That thinking makes sense mainly only if there is a destination in mind that Green 'wants' to go to.
Part of that is because of his player option ('means he can for now control the salary commitment of any team he goes to which makes it difficult to set a trade price on him) and the other part of that is that Turner and Green both have the same agent (David Falk).
It seems unlikely that Falk would bring Turner in to the Celtics to enable moving Green unless Green wanted to be moved.
All that is not to say that Green wants to be moved or not. Just that I wouldn't necessarily assume that replacing Green is why Turner was brought in.
Even if we aren't talking about a trade and are just talking about Turner being a replacement after Green walks if he opts-out, that still would not be in Falk's client's best interests (if Green didn't want to leave to another team as FA) because it decreases Ainge's incentive to bid for Green.
To this point, and probably for the near future, Green has made and will make Falk a lot more money than Turner. Falk has himself spoken at length in interviews about how strongly he emphasizes loyalty between himself and the players he represents. So it would seem odd for him to want to set up one of his players to the disadvantage of another. So that implies that maybe the plan for Turner is not in that direction.
Though I fully admit, I'm still scratching my head on it.
Of course, if Danny were easy to figure out, he'd suck as a GM.