Most promising thing I've seen from Green has been a couple of blocks and a defensive possession that he took Nash on.
Other than that, it's a bit far fetched. That said, Perk is quite hurt at the moment, and I'm going to give the benefit that Green should be better defensively for us this year.
Also, while I think Perk is a very good defender, he's quite a limited defender... limitations he uses to the fullest. For example, he's benefited a great deal from centers that are also very limited offensively.
But his help defense has been excellent in many regards. I'd say his defensive job was one of the most important aspects of stopping LeBron in many of our playoff series, particularly the 1st one (when too much credit was incorrectly given to Posey).