Morales was out of the lineup for Wednesday's series finale with the Blue Jays, and Yost wasn't sure when Morales would return to the lineup.
"Just day to day," Yost said. "It just locked up after last night's game. He thought it would get better overnight, but it didn't."
Morales, the reigning American League Player of the Week, played five straight games in right field last week, his first outfield duty since 2008. He is hitting .423 on the trip (11-for-26).
"It's pretty sore," Morales said in English before shaking his head.
Who starts Sunday?
Right-hander Chris Young has surrendered 14 runs in his last three starts covering 8 2/3 innings, and as of Wednesday, was still listed as Sunday's starter in the final game before the All-Star break.
Yost said he hasn't made a decision, though, on that starting spot.
"I haven't even thought about it," Yost said. "I've got too much else going on. We've got time to decide. We'll get together and discuss it when we get back to Kansas City [Thursday]."
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.