1) Rookie - You never want to throw rookies into the fire unless you have to. Davis is an exception, and Rivers will only start if Gordon is still injured.
2) Aminu knows the system better than Miller, and he is a better overall defender/rebounder. Sure, Miller can hit the open J, and I think that is why he will eventually take over, but he has to get the mental aspect down first.
3) Aminu has experience as a starter, so he knows what to expect from the elite SFs in this league. Miller still has to learn, and that relates to point #1. You don't want to shatter Miller's confidence at such a young age. Start him slow, and give him more minutes as the season progresses.