Marsh is moving to the bullpen since Michael Lorenzen (hamstring) is returning to Kansas City's rotation Wednesday against Washington, Anne Rogers of MLB.com reports.
Marsh is scheduled to start Saturday's game against the Twins in Kansas City.
Marsh (7-8) took the loss Sunday against the Astros, allowing three runs on four hits and a walk while striking out five over five innings.
The Royals recalled Marsh from Triple-A Omaha to start Sunday's game against the Astros.
The Royals recalled Marsh from Triple-A Omaha on Monday.
The Royals optioned Marsh to Triple-A Omaha on Wednesday.
Marsh appeared out of the bullpen in Friday's 7-1 win over the White Sox, allowing two hits and striking out one over one scoreless inning.
Marsh (6-6) took the loss against the Rays on Thursday, allowing five runs on six hits and three walks over three innings while striking out one.
Marsh came away with a no-decision in Thursday's 4-3 win vs the Yankees. He threw seven shutout innings, allowing one hit and two walks while striking out seven.
Marsh (5-3) got the win vs the Mariners on Saturday. He went five innings, giving up four runs on seven hits while striking out eight.