The club plans to promote No.3 prospect Eric Skoglund, and start the left-hander Tuesday against the Tigers. It will be Skoglund's Major League debut.
Duffy, who signed a five-year, $65 million extension over the offseason, is 4-4 with a 3.54 ERA in 11 starts this season. He was the Royals' Opening Day starter.
Skoglund, whom the Royals drafted in the 3rd round in 2014 out of Central Florida, is 2-3 with a 4.53 ERA in Triple-A Omaha this season. The 24-year-old is considered one of the most polished lefties in the Royals' system.
Joe Trezza is a reporter for MLB.