"I wouldn't even call it an injury," Yost said. "Just some tightness. We don't have much concern over it, either."
Duffy left the game in the third after throwing two fastballs, both at 92 mph. He threw 32 pitches altogether. He was scheduled to throw 95.
Yost and ahtletic trainer Nick Kenney first visited Duffy at the mound in the second inning, but Duffy stayed in the game and got three straight outs.
"The pain is gone as I'm talking to you," Duffy said. "I'll pick up a ball tomorrow and we'll quote-unquote test it out, but I already know it's fine."
Duffy said he first felt some tightness two weeks ago while doing some pickoff drills.
"I felt a pop but [the tightness] went away right away," Duffy said. "I was in the mid-90s next outing. Same thing happened today, and all the tests afterward were negative. It's just precautionary. I'm fine."
The Royals' bullpen situation got a little clearer as they optioned right-hander Kevin McCarthy to Triple-A Omaha after Saturday's game, which ended in a 4-4 tie with the split-squad D-backs.
McCarthy was an early-camp favorite to make the Opening Day roster but posted a 6.30 ERA in eight appearances.
Yost has indicated he will carry eight bullpen pitchers. Kelvin Herrera, Brandon Maurer and Justin Grimm are locks. Rule 5 acquisition Brad Keller, Blaine Boyer and Wily Peralta seem almost sure bets as well, though Peralta's shaky spring has created some doubt. The last two spots could come from this group: Eric Skoglund, Tim Hill, Brian Flynn (out of options) and Rule 5 acquisition Burch Smith.
Keller, trying to land a bullpen spot, continued his strong spring with two scoreless innings Saturday, lowering his ERA to 2.70. Hill, still with an outside chance, lowered his ERA to 2.57 with a scoreless ninth.
• Infielder Adalberto Mondesi, who has been battling right shoulder impingement, made his first start in the field at second base since early March.
• Outfielder Bubba Starling is still out with a strained oblique but began throwing Thursday.
Right-hander Ian Kennedy will start for the Royals in their final Cactus League game Sunday at 3:05 p.m. CT against the Cubs. Kennedy is expected to start the second game of the regular season next Saturday against the White Sox.