"We're going to go day to day with it," Yost said. "I don't know. With the off-day coming up Monday ... we just got to get a handle on it."
Perez said he aggravated the injury a few games ago. But Perez added that it hurt worse after striking out against Indians reliever Joe Smith late in the game on Thursday. Drew Butera replaced Perez behind the plate.
"These guys are pretty honest about [injuries]," Yost said. "He said, 'I can catch. I can't swing.'"
Perez was heavily wrapped in ice around his midsection after Thursday's game and he spent much of the pregame on Friday getting treatment. The intercostal is a group of muscles inside the rib cage that help support the chest wall.
"Just rest today and see how I feel [Saturday]," Perez said.
Perez said he certainly is getting frustrated with the injury, which won't heal completely until the offseason.
"When this first happened to me six or seven weeks ago, it's just been trying to stay healthy ever since," Perez said. "I don't know ... it's 16 or 17 more games [in the regular season], we'll see what happens when the season is over."
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.