"MLB called and said it was a violation of the taxi-squad [rule]," manager Ned Yost said.
"This is where common sense needs to come into play. A violation of the taxi-squad rule is if you've got a player hanging here that's available -- you've got a guy here three or four days in case something happens, that's a clear violation. This kid's not available, but they said it's a violation of the taxi squad -- get him back."
Duffy worked six innings, held the Tigers to one hit and outpitched Justin Verlander in the Royals' 2-1 first-game victory.
"He was going to fly back [to Kansas City] with us," Yost said. "My thinking is, 'OK, the kid pitched his butt off, he's unavailable, let him stay here, throw a side session, he can fly back with us and go to Omaha.' A little bit of a reward."
But no. Duffy was off to Memphis, where he'll sit on the Storm Chasers' bench. He's scheduled to pitch for them on Wednesday, after they return to Omaha.