While Rozier hasn't played as well as he had in preseason, I think the reason he is sitting is that Stevens realized that playing Green as the 4th guard was the key to getting more out of Smart on offense, and causes more problems for the defense overall.
Smart can bully point guards, but he has a harder time pushing around many of the two guards he was matching up against. Green gives you off the bench scoring like Rozier, but he's quite a bit taller, and forces teams to move their bigger guard off Smart.