Perez explained that his lower right leg has been struck multiple times when he was behind the plate in the last week or 10 days.
"Foul balls or when I try to block the ball, sometimes they get behind the shin guard and hit me in the same spot," Perez said. "It's not a big deal. I can play, but they want me to give it a rest today."
Yost believes that Perez will be back in the lineup on Monday night at Yankee Stadium.
"It's evident when he runs, how sore it is. It's not anything he couldn't play through, but the more he plays on it, the sorer it gets. So you pick a spot every three or four days to try to get him over the hump," Yost said.
Backup George Kottaras was the starting catcher against the Athletics, his team for the second half of last season and in the playoffs.
"I didn't even think about that factor until I got here this morning and thought, 'Hey, this is a chance to give George a chance to play against his old team,'" Yost said. "You always enjoy those opportunities when you get to go back and play against your old team."
Kottaras belted a home run in his first at-bat against former batterymate A.J. Griffin and later drew two walks in the Royals' 10-4 loss.
Although this was just Kottaras' 15th start, Yost likes the job he's done.
"He's prepared every single day, he gives you a professional at-bat and has done a great job behind the plate in the games that he's played. He receives the ball well, blocks the ball fine and calls a really good game," Yost said. "He's a good complement to Salvy."
Dick Kaegel is a reporter for MLB.com. Kathleen Gier is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.