Duffy (1-4, 4.85 ERA) was sent back to the minor leagues before the All-Star break so he could pitch on regular rest. He allowed four runs in six innings for the Storm Chasers last Thursday.
The 22-year-old rookie, who is winless in his last four starts, has nonetheless established himself as one of the most promising pitchers in the Royals' beleaguered starting rotation. He allowed just two earned runs in four of his last five starts at the major league level.
The Royals sent reliever Everett Teaford back to Triple-A after Monday night's game to clear space for Duffy, who manager Ned Yost has said could stay in the rotation the rest of the season.