I am not sure it has been mentioned yet, but this signing was actually genius, in how it was put together.
Danny was able to essentially eliminate the issue of Green losing his Bird rights, if he is traded before the deadline this year. By giving him the raise that he did, rather than the QO, he allowed Green to be able to get a non-Bird right contract from a team that trades for him, starting at over $10 million a year, which is probably no less than what he would be looking for anyways.
So, if the trade deadline comes around, and Danny finds a deal that makes sense for Green, he will be much less likely to veto it, and the team that trades for him will be more likely to give up more value, because they will be able to resign Green, or give him an extension that is close to market value, which might not have been the case with the QO.
Obviously, the best case scenario is that Green performs, and stays with the C's, but this was a clever move by Ainge (and Green's agent) to keep an option open for later in the season.