Stewart was optioned to Triple-A Oklahoma City on Wednesday, Eric Stephen of TrueBlueLA.com reports.
As expected, Stewart was shuttled back to the minors after serving as the 26th man for Tuesday's doubleheader against the Cubs. The 26-year-old allowed the game-winning run in the 10th inning on a pair of hits and a walk before recording an out, raising his ERA to 6.11 through 17.2 innings this season.