The Athletics selected Miller's contract from Triple-A Las Vegas on Wednesday.
He'll make his major-league debut Wednesday against the Cubs and will have a "soft cap" of around 80 pitches, manager Mark Kotsay told Jessica Kleinschmidt of TheComeback.com. Miller boasts a 19:0 K:BB over 8.2 innings between Las Vegas and Double-A Midland over his first two minor-league starts this season.