The Diamondbacks optioned Miller to Triple-A Reno earlier this month, ArizonaSports.com reports.
Miller split time between Reno and Double-A Jackson last season, logging a 2.93 ERA and 94 strikeouts while holding opposing hitters to a .190 average over 70.2 frames. Those results prompted the Diamondbacks to protect Miller from the Rule 5 draft and add him to the 40-man roster over the winter, but the lefty wasn't under any serious consideration for an Opening Day assignment with the big club. He'll likely assume high-leverage work with Reno and should earn his first callup to the majors later this season.