Honestly, probably nothing...unless it's a massive highway robbery type deal (in Boston's favor).
Yeah I know Green is really good, but he just reminds me far too much of Rajon Rondo. He's that guy who does a lot of things well, is a triple double machine, and yet will never be the #1 guy on a top team. He's basically a hyper role player.
Like Rondo he's also a big arrogant, a big of a troublemaker, and just generally a wee bit of a head case - actually a massive head case.
Then to add to this, I'm just not sure his game actually fits us right now. I'm not a huge fan of the "small ball" direction the league is taking, and I just don't like the idea of a Green/Horford Front court. I think the level of rim protection would be woeful, the rebounding would be not much better, and our front court would be spending too much time sitting on the perimeter. Plus I just don't like the idea of a 6'7" + 6'10" frontcourt.
Call me old fashioned, but I think a guy of Horford's play style needs a more traditional interior-oriented big many next to him for optimal effect. Even if Green wasn't a total knob-head (which he is), I still wouldn't love the idea.