Fortunately, we don't have to worry about Green walking on his own terms any time soon -- he's signed through 2014-15. At that point, Manu will almost certainly have retired, and Green could be our star SG rather than our first-off-the-bench SG who just happens to start because we play Manu off the bench.
More likely, between now and then, Green comes back down to earth, settles into his contract a bit more, and continues to be an excellent role player.
The only problem with trading him is the salary-matching rule. Green gives us more for $4 million than any other $4 million dollar player would give us. We don't have any big expiring contracts to package with him, unless we sign-and-trade Tiago along with him. Of course, a sign-and-trade of Tiago is quite possible in the off-season given that he's a RFA, but honestly I don't think packaging Green with him would add much value to whatever we'd get in return.
I think we've got a nice position with Green. He isn't going anywhere for at least two seasons unless we trade him, and he brings us far more value than any trade for him would get us. I predict he'll be around at least through the end of our three-peat in 2015.