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.
Using basketball-reference, I listed all of the players who played a total of at least 500 minutes and averaged at least 10 minutes a game last season, and then sorted the list by Win Shares Per 48 minutes.
I counted 47 players who I would consider to be "wing" players ahead of Green on that list.
Furthermore, using the same criteria, among players listed as G / G-F / F-G or F, here is how Green ranks in the following categories:
Points per 36 minutes: 46th
Rebounds per 36 minutes: 60th
Assists per 36 minutes: 77th
Steals per 36 minutes: 79th
Free throw attempts per 36 minutes: 39th
Blocks per 36 minutes: 14th
TS%: 46th
PER: 79th
So it might actually be charitable to say that Green is "average" in many areas. The only place he stands out at all is in the blocks category.
Trust me, I realize just how many crappy players are getting starter minutes (or close to it) in the NBA. That doesn't mean that Green is an "average" player for his position in terms of production.
There are lots of players who just do more to help their teams win; not all of those players are getting starter minutes, however.
Tayshaun Prince is an interesting comparison because, like Green, he has never been especially productive in statistical terms. He did, however, play superb defense (he made 4 All-Defensive 2nd Teams), and stepped up his game in the playoffs. Also, Tayshaun has a career offensive rating of 109 with a defensive rating of 107.
Green, by comparison, has that same defensive rating of 107 with an offensive rating of 103. Which means that over his career, Green's teams have been worse with him on the floor than without.