"Another left-handed arm. He's been doing well down in Omaha, been closing some games out. We wanted some more left-handed protection." manager Ned Yost said.
Bueno joins Tim Collins to give the bullpen a second lefty. Will Smith, a left-hander who was returned to Omaha on Wednesday, has to remain in the Minors for 10 days.
That would be the case for Duffy as well, except he could return before that as the 26th man permitted for a doubleheader, which the Royals happen to have a week from Friday at Detroit. Although Yost is not revealing any plans in advance, that's certainly an option.
Meanwhile, Duffy could make a start for Omaha with limited pitches between now and then.
"I know they have confidence in my stuff. They wouldn't have brought me here if they didn't," Duffy said. "I'm really lucky and blessed to say I was part of this team in one way or another."
In his return from Tommy John surgery on Wednesday night, Duffy started a 5-2 victory over the Twins and went 3 2/3 innings, giving up two runs.
"You just wanted him to get out and get it over with. It's like your first Major League start. I wanted to get past it because when you go through what he's gone through and what [Felipe] Paulino has gone through, that's a big start for you," Yost said.
"You come back, you've battled your way back. You know there's going to be a lot of emotion, you know all the hard work you've put in for the last 14, 15 months -- and this is why. You get the opportunity to go back out and pitch a Major League game."
Duffy made 31 pitches in the first inning and exited after 93 pitches.
"His stuff was great, his curveball was off, his command was off, but a lot of that had to do with overthrowing and being hyped up in the moment of everything that was involved with it," Yost said.
Bueno, 32, had a 3-3 record, a 2.93 ERA and one save in 31 games for Omaha. In his last six outings, his ERA was 0.87. Signed as a free agent prior to the 2012 season, Bueno relieved in 18 games for Kansas City last year, with a 1-1 record and 1.56 ERA.
Moore, 29, is out for the season after undergoing surgery because of a torn groin adductor. He was with the Royals while Salvador Perez was on bereavement leave this year and played five games, going 3-for-10. For Omaha, Moore batted .191 with eight homers and 23 RBIs in 41 games before being injured on July 19.