Medlen was on a rehab assignment with Triple-A Omaha after suffering right rotator cuff inflammation that landed him on the disabled list on May 12. He left Tuesday's game after throwing a breaking ball on his 49th pitch and returned to Kansas City for testing.
In six Major League starts this year, Medlen had gone 1-3 with a 7.77 ERA.
The Royals did get some positive news on the injury front Thursday when Kendrys Morales (stiff back) took some indoor swings in the cage and felt good enough to take his place in the lineup for the opener of the Seattle series. Morales was the designated hitter and Yost penciled him in at No. 3 in the order.
Robert Falkoff is a contributor to MLB.com based in Kansas City and covered the Royals on Thursday. This story was not subject to the approval of Major League Baseball or its clubs.