Miller was optioned to Triple-A Reno on Thursday.
This move comes as a surprise, not only given Miller's contract situation, but also due to the injuries to other starters such as Zack Greinke and Rubby De La Rosa. That said, Miller has been one of the more disappointing offseason acquisitions in baseball this season, struggling his way to a 7.14 ERA with 50 strikeouts and 34 walks over 69.1 innings of work. He'll head back down to Triple-A to iron out his issues for the time being. The Diamondbacks will make a corresponding roster move Friday.