Chen is making progress in his recovery from a bulging disk in his back but he's not far enough along to return to the rotation by the May 17 date that manager Ned Yost had hoped.
Back in Kansas City, Chen is undergoing treatment and playing catch but he's not yet gone out to pitch on a Minor League rehabilitation assignment.
So Duffy, who was passed over in the latest turn of the rotation because of last Monday's open date, will make his third start as Chen's replacement.
Duffy went four innings in his first start, giving up one run on two hits to the Tigers, and then went six innings at San Diego, again with one run on two hits. Both games turned out to be losses but Duffy's showings were encouraging.
"He's been doing fine," Yost said. "Our pitching's been fine, we just need to get the offense going. [Yordano] Ventura gives up three runs and it felt like it was 19."
That was a reference to Saturday night's 3-1 loss when the Royals came up with only four hits.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.