He was recalled from Triple-A Omaha and second baseman Johnny Giavotella was optioned to the same club. That brings the Royals' pitching staff back to 12, including seven relievers.
Duffy, a prize left-handed prospect of long standing, will operate out of the Kansas City bullpen.
"We're trying to put the best arms we've got in the organization [here] right now and he's one of the best arms we've got," manager Ned Yost said.
Duffy was the center of a Spring Training debate among Royals officials -- should he begin the season in the KC bullpen or go to Omaha and get lengthened out as a starter? Ultimately the Omaha option won out -- Duffy would start for the Storm Chasers and be ready in case the KC rotation needed help.
That lasted exactly one game. Duffy pitched six innings, gave up two runs and earned a victory. He was held out of Friday's start pending his recall.
"We've been going with 11 pitchers, back to 12, back to 11. You've got issues with rosters -- when you send guys down, they must be there for 10 days," Yost said. "You can't recall 'em unless somebody goes on the DL. And he threw the ball really well."
During Spring Training, 2013 relief standout Luke Hochevar was lost to Tommy John surgery.
"You always feel a loss like that, especially somebody as good as he was," Yost said.
Just five days ago, the Royals had two left-handed relievers go on the 15-day disabled list. Three days ago the only remaining lefty reliever, Donnie Joseph, was sent back to Omaha so that Giavotella could be called up as a fill-in for injured second baseman Omar Infante.
Meantime, right-hander Louis Coleman returned to the bullpen from the disabled list and rookie righty Michael Mariot was called up from Omaha.
In Friday night's game, Infante returned from his bout with a baseball to the jaw and held up well. So Giavotella's presence was less vital and he left after playing just one game.
Duffy becomes the lone lefty in the bullpen.
"And we needed some length, too," Yost said and referred to Friday night's 10-1 loss to the Twins. "[It)] was one of those kind of nightmares when your starting pitcher goes 3 2/3 [innings] and you've got an 11-man staff, and you're already down 4-1 and you're trying to hold the game so you can crawl back in and get to your back end relievers. And then, we had to bring [Kelvin] Herrera in when we really didn't want to, but we couldn't throw strikes and pitch counts got up and ..."
Yost found himself a bit short-handed in pitching.
If necessary, Duffy could provide multiple innings of relief as he did on Saturday.
Collins and Bueno might be ready to return within about 10 days so could Duffy's stay be temporary?
"Not necessarily, we'll play all that by ear. It depends on how he does," Yost said. "We're trying to win baseball games."