This will not be a spot start for Medlen, whom Yost expects to give several starts down the stretch.
"It gives us another option [for the postseason] to see what direction we want to go," Yost said. "Obviously there's been with Jeremy some production [issues], but it's more about finding out about Medlen."
Guthrie has a 5.65 ERA and opponents have hit .313 off him.
Medlen, who has had two Tommy John surgeries, was signed by the Royals in the offseason with the hopes that he could become a starter in the second half. In seven appearances, he has a 2.51 ERA. He threw three shutout innings on Thursday against Boston.
"I'm excited, because, obviously, I've worked back from just wanting to throw pitches again in the big leagues to wanting to be in the rotation again," Medlen said. "It has taken a lot of hard work and I've had to show I can compete. Unfortunately, Guthrie has struggled a little bit in his last couple of starts, so hopefully I can step in and help us."
Medlen learned the news that he would join the rotation from pitching coach Dave Eiland on Thursday. On Friday, Yost told Medlen to make it official.
Medlen likely will be on about a 60-75 pitch count.
"I told [Yost] not to tell me how many pitches I get, because it's the last thing you want to worry about trying to get the Baltimore Orioles out," Medlen said. "I'm going to go do what I've always done: Throw strikes or attempt to throw strikes. I'm not afraid to give up hits, because there's always a way to get out them. All pitchers give up hits, but the good ones get out of jams."
Jeffrey Flanagan is a reporter for MLB.com. Follow him on Twitter at @FlannyMLB. This story was not subject to the approval of Major League Baseball or its clubs.