Miller (back) was transferred to Double-A Reading on Tuesday to continue his rehab assignment, Todd Zolecki of MLB.com reports.
The 22-year-old was sent to Single-A Clearwater on Saturday to begin the rehab assignment and will move up to the Double-A level after going 1-for-2 with a walk and a double in one game. Miller has been on the shelf all season due to a back injury, but he could see action with Triple-A Memphis before the end of the regular season in a couple weeks.