I don't think the question is whether Casspi will be OK as the sixth man off the bench. I think he's proved he's more than capable. The bigger question is whether Greene can do whats required to remain the starter, and that has nothing to do with Casspi. You could keep Casspi as the sixth man and have Cisco start at SF. Or you could give Wright a shot.
I'm by no means calling for Greene to be benched. Its way too early for that. But at some point he has to start playing with some fire in his belly and quit disappearing in games. He needs to become an important piece of the puzzle.
Casspi's biggest fault, is that sometimes he trys to do too much. Greene's biggest fault, is that too often, he trys to do too little.