Given his inconsistency and lack of efficiency, I'm not convinced Green is really "average" in the sense of overall value and production. He lacks an elite skill and he doesn't score consistently enough or efficiently enough to truly overcome that fact. I'd rather have Thabo Sefolosha starting for my team, for example, despite the fact that he's never averaged close to 16 points a game.
You don't have to have one elite skill to be considered at least an average NBA player. Guys like Scottie Pippen and Tayshaun Prince did just fine by being jacks of all trades, masters of none. There's no blueprint.
In any case, I also consider Thabo Sefolosha at least an average NBA starter, despite the fact that he isn't a prolific scorer. I just don't think you realize how many truly horrible players in the NBA are getting starter or close to starter minutes. I mean guys such as Singler, Gee, Zeller, Ariza, Aminu, you name it.