Stewart was optioned to Triple-A Oklahoma City on Tuesday, J.P. Hoornstra of the Los Angeles Daily News reports.
Stewart will head back to the minors in order to clear a roster spot for Tuesday's starter, Rich Hill (finger), who was activated from the 10-day disabled list in a corresponding move. The 26-year-old owns a 2.41 ERA across four starts (18.2 innings) with Oklahoma City this season. He'll remain one of the team's top options for a promotion when additional bullpen help -- or potentially a spot starter -- is needed in the future.