Left-hander Bruce Chen, who was originally scheduled to start on Wednesday, will face the Twins on Thursday afternoon instead.
Since undergoing Tommy John surgery on June 13, 2012, Duffy has been on the road back. He made his first Major League start since the elbow surgery on Aug. 7 against the Twins, giving up two runs on six hits in 3 2/3 innings. He returned to pitch on Aug. 16 at Detroit, shutting out the Tigers on one hit over six innings.
Since then, Duffy pitched once for Triple-A Omaha and went five shutout innings against New Orleans. For the Storm Chasers this year, Duffy was 3-0 with a 4.08 ERA in 12 games.
"We haven't given up on making the playoffs," Yost said. "We want Duffy on our playoff roster. And if we do make the playoffs, we're probably going to go with three starters and Wade is going to be in the 'pen at that time, so we might as well get him acclimated the last month."
Yost also pointed out that Davis, who has pitched 125 1/3 innings this season, is already 55 innings over the 70 1/3 innings he logged last season, when he worked out of the Tampa Bay bullpen.
Davis has a 6-10 record and 5.67 ERA in 24 starts.
"He had some really good games and some really bad games and not a bunch of games in between that," Yost said.
The Royals also want to build up Duffy's innings with an eye toward next season. So far, in the Minors and Majors combined, Duffy has thrown 78 2/3 innings.
When Duffy and Luke Hochevar, who is on paternity leave, arrive on Wednesday, the Royals will have to drop two players off the 25-man roster.
"We're looking at a bunch of different scenarios, and it all revolves around our playoff roster," Yost said.