The Royals designated Duffey for assignment Friday.
The right-hander was added to the Royals' roster in late April and posted a 5.00 ERA, 1.67 WHIP and 10:8 K:BB over nine innings during his month in the majors. Duffey was removed from the 26-man and 40-man rosters to clear a spot for the return of Carlos Hernandez (shoulder).