Jeff Green wasn't efficient as a 3rd option, even with Durant and Westbrook getting all the attention and him being defended by much slower players.
OKC without Durant? In that case, Harden would become their 2nd best scorer and Green would still be a 3rd option - just in a 20/30 wins team. Maybe a 18ppg player... but an even more inefficient one.
With the Celtics he's being more efficient because he's playing more as a role-player as he doesn't have to construct his shots so often (barely ever).
Anyone who believes he can be an effective (prolific and efficient) top-3 option in a good team is extremely delusional.
The idea that he isn't being used enough is bogus. His usage rate is 20.1%, which is 4th in the team, below Pierce, Garnett, Glen Davis. He uses more possessions than Ray Allen. In fact, he should be used less.
Finally, the biggest knock on Jeff Green during his AAU days playing for Curtis Malone was for being... too passive. He was too passive in college and during his entire NBA career. He's a soft and timid player and has always been. Blaming Rondo or Garnett or Doc Rivers for his playing ethos is beyond absurd.