The Royals recalled right-hander Jason Adam from Triple-A Omaha to take Kennedy's place on the 25-man roster.
Kennedy left Tuesday's game against the Minnesota Twins with an oblique strain after three innings. He was sidelined in June with an oblique strain, but said the latest injury, while on the same side, is in a different spot.
He was making his first start since returning from the previous injury
"All week, the last 10 days, everything has been getting better,'' Kennedy said Tuesday. "Felt great all night, and then all of a sudden I felt something at the bottom of my ribs. ... It's just aggravating."
Kennedy is 1-8 this season with a 5.13 ERA.
--Field Level Media