Manager Ned Yost termed the injury "medial left-elbow tightness," and Duffy was to return to Kansas City on Sunday night to undergo an MRI on Monday.
Duffy, 23, said he felt fine during his pregame bullpen session before feeling tightness on the inner part of his pitching elbow. Three batters later, and 13 pitches in, catcher Humberto Quintero saw Duffy flick his arm and yelled to bench coach Chino Cadahia and Yost, who visited the mound with head trainer Nick Kenney. After a brief discussion, they removed Duffy from the game.
"I came out and [he said] 'I'm all right, I can pitch through it,' but as soon as he said it was in [his] elbow, I said that's it," Yost said. "We need to get that checked and see what's going on there, and go from there."
Duffy said he wasn't sure if Sunday's tightness was similar to what he felt last month, when he was scratched from an April 29 start against the Indians. He said his biggest concern in leaving was letting down his teammates who, behind 5 2/3 innings from Luis Mendoza, earned the win.
"I'm pumped to see my boys pull through today," Duffy said. "It would've been a real tough day if they didn't. They did and it was awesome."
Duffy, the club's third-round pick in the 2007 Draft, also wasn't upset following the game, opting instead to reserve judgement until finding out the MRI results.
"I can't be because I don't know what it is," Duffy said. "I just have to put it in God's hands, and give it to Him and see what it is. We'll adjust when we know, but until we know, we can't stress about it too much."