"He's slowed up and in extended [spring]," Royals assistant general manager J.J. Picollo said. "He needs more time before he's ready to get out. [His] shoulder has been a little erratic -- some great days and some so-so days, so we slowed him up."
Zimmer made three appearances in big league camp this year and gave up just one run in six innings. But in his last Grapefruit League outing, Zimmer's velocity dipped from 94-95 mph to 88-89 mph. He attributed the loss of velocity to the typical "dead arm" pitchers can get at that time of camp.
The Royals do not have a set timetable for Zimmer to be sent to Omaha.
"He's day to day right now," Picollo said. "He's throwing side [sessions] starting next week, and we will determine it day to day."
Zimmer, who was Kansas City's first-round pick (fifth overall) in the 2012 Draft, has battled several injuries during his brief career.
The Royals' level of concern for his latest injury issue?
"We're more concerned for his psyche than his health," Picollo said.
Jeffrey Flanagan is a reporter for MLB.com. Follow him on Twitter at @FlannyMLB. This story was not subject to the approval of Major League Baseball or its clubs.