Kennedy will be re-evaluated on Friday.
After getting Jose Abreu on a flyout to center to open the fifth inning, Kennedy winced after delivering a pitch to Avisail Garcia. Trainer Nick Kenney, pitching coach Dave Eiland and Yost visited the mound and the decision was made to take Kennedy out.
"It feels fine right now, walking," Kennedy said. "But extending fully, I felt it on the popup before. Then, I felt it pull to the next hitter. It's on the push-off leg."
Kennedy injured his other hamstring toward the end of Spring Training in 2016. He didn't miss a start to begin that season, but he was pushed back a few days.
"It felt like a groin because it's right on the inside," Kennedy said. "They have told me to not test it. It stinks."
Kennedy, who was replaced by left-hander Mike Minor, gave up six hits, five runs (four earned) while walking one and striking out three. He gave up home runs to Abreu in the first and Matt Davidson in the second.
"I was missing all over the place," Kennedy said. "Fastball right down the middle to Abreu. Bad changeup to Davidson. Fastball command was all over the place. Just a bad day."
Kennedy simply wasn't sure how long the injury will keep him out.
"It'll be up to Nick and Ned and Dayton [Moore] to decide," Kennedy said. "Like I said, we'll see how I feel tomorrow."
Jeffrey Flanagan has covered the Royals since 1991, and for MLB.com since 2015. Follow him on Twitter @FlannyMLB. This story was not subject to the approval of Major League Baseball or its clubs.