Zimmer is currently being listed as suffering from a fatigued arm, Jeffrey Flanagan of MLB.com reports.
Zimmer has only pitched 36.2 innings this season, missing time earlier in the year with a shoulder injury. With that being said, it wouldn't be surprising to find out there is more to the story than the Royals are currently letting on, considering he threw 89.0 innings in 2013 and 64.0 in 2015, suggesting the number of innings he's pitched this year is not an unfamiliar work load.