Duffy was taken out of the game because of cramping in his left calf after the Cardinals' Pete Kozma singled with two outs in the fourth inning of the Royals' 5-4 loss.
"It's just a cramp, it's not a strain or anything," manager Ned Yost said.
When he left, Duffy had registered nine consecutive strikeouts while getting 11 outs as the Cardinals seemed largely baffled. Not so baffled, however, that they couldn't score two runs with three hits and a walk in the first inning.
Yet, after Colby Rasmus flied to left for the Cardinals' first out, the next nine outs came on strikes, all but two swinging. That string was broken when opposing pitcher Jaime Garcia grounded out to short. That was followed by Kozma's single to left and Duffy's departure.
"It just grabbed me a little bit. I didn't really expect to come out but it's just one of those things -- you've got to err on the side of caution," Duffy said.
"Growing up, I used to wonder why pitchers would come out with blisters -- that's not who I am. I'd be out there with a broken leg. The last thing I want to do is use up our bullpen the way I did today. It's just unfortunate it happened the way it did, but I'll hydrate really well next time."
The game began in 83-degree heat laced with St. Louis' famous humidity.
The glut of strikeouts came on 90 pitches (56 strikes) in Duffy's 3 2/3 innings.
"In terms of being a big league pitcher, he got away with a lot of pitches up," Yost said. "He was behind in the count. He needs to take that stuff, lower it in the strike zone, keep his pitch count manageable, which he didn't do. But, yeah, he's got all the makings of being a really good Major League pitcher. He just needs time and experience at this level to work it out."
Duffy said he'll take the trainers' advice and stay off the leg on Monday, but he's OK.
"I'm fine, I can go out and play catch right now," he said.