Royals general manager Dayton Moore indicated he wasn't sure as of Wednesday morning whether Zimmer had been evaluated medically. Zimmer was scheduled to throw three to four innings on Tuesday.
Zimmer has battled arm and shoulder issues during his Minor League career since the Royals selected him in the first round of the 2012 Draft.
Zimmer experienced shoulder fatigue during Spring Training and was held back in extended spring to increase his arm strength. He has pitched three times in the Minors since leaving extended spring, posting a 1.59 ERA.
Moose to Double-A Third baseman Mike Moustakas, on the disabled list because of a broken bone in his left palm, was the DH on Wednesday for Double-A Northwest Arkansas in a rehab assignment.
Royals manager Ned Yost said Moustakas would play third base on Thursday and then be re-evaluated. Moustakas is expected to rejoin the team in Chicago on Saturday.
Mills 26th man Right-hander Alex Mills, the team's No. 14 prospect according to MLBPipeline.com, was selected as the team's 26th man for the Royals' doubleheader on Wednesday. Yost said they also considered left-hander Matt Strahm.
Mills was 1-1 with a 2.36 ERA in seven starts for Double-A Northwest Arkansas.