Green looks to be a serviceable player, but as others have mentioned, his age puts a ceiling on his potential. Even if he might be better than other guys at the very end of the roster, NBA teams put a premium on what could be. We have already signed Wanamaker, Theis, and Poirier as older players with little-no NBA experience (not so much in Theis' case, but once upon a time...) so there's only so much we can do with so many rookies, as well.
If Green was given a real opportunity, I believe he could be a rotation bench player on a non-contending team. There are a lot of talented individuals out there; unfortunately not all of them are destined for NBA prime time (even if some are more deserving than others - see Yabu, James Young, Fab Melo, etc).