I'm not completly giving up on miller but i am worried for sure.
yes he is 21 and in AAA having a lot of success before, but his velocity is down, his secondary stuff is not very good because he hasn't worked on it and used it much in game. His command is iffy this year (probably due to secondary pitches), he isn't being efficient, he is much more hittable and his FB is getting smoked for a bunch of HR's.
hopefully working A LOT on his off speed stuff, getting in better pitching condition to go deeper into games and gain a little velocity back, and better command will get him back to where he was.