The elimination of the four pitchers narrows the in-camp field for the seventh and last bullpen spot to left-hander Francisley Bueno and right-hander Jon Rauch.
Ever since Yordano Ventura won the fifth starting job, the Royals had been debating whether it would serve the club best if Duffy was working against Major League hitters out of the Royals' bullpen or conditioning himself as a starter for Omaha.
"We looked at it, and we just thought it's better to have him down there building up than it was in our 'pen right now," Yost said. "We think he's going to be a big part of our pitching staff this year. He's been a starter his whole career, and we think that it's going to be more beneficial for him to do that."
Duffy, in six Cactus League games and 11 innings, gave up 16 hits, 14 earned runs, six homers and six walks to the tune of an 11.45 ERA. The decision was made after he gave up two walks and a home run in one inning against the Rangers on Saturday.
"It wasn't the home run, it was a combination of things," Yost said. "The two walks before the home run were as big as the home run. It wasn't: 'He gave up two walks and a home run, let's send him to Triple-A.' He's better off being a starter where he can get down there, get in his rhythm, get to where he can dominate that league like he can with his stuff."
Yost noted that Duffy's fastball reached 98 mph on Saturday, but at the same time, he got an 0-2 count on a batter and then walked him. This was his second relief outing since Ventura bumped Duffy from the rotation competition.
"Does he need to get big league hitters out? Yeah, he does," Yost said. "But does that overtrump us needing a guy ready in case we need a starter? If he came in and absolutely blew the doors off as a reliever, OK, we'd look at it for about six weeks and then we probably would've have sent back down anyway to get built back up."
With the Royals seemingly on the cusp of being a serious contender in the American League Central, the time for developing players at the big league level is over, in Yost's view.
"Would we be sending Danny Duffy down two years ago? Naw. We'd let him figure it out up here," Yost said.
Duffy, 25, spent last season rehabbing from Tommy John surgery. He made five starts for KC and went 2-0 with a 1.85 ERA. Before the surgery in 2012, he made 26 starts over two seasons for the Royals; his overall record for them is 8-10 with a 4.75 ERA.
"We think his delivery is as good as it's been since he's been here," Yost said. "It just goes back to getting that edge and going back out and attacking with his great stuff to where he can dominate hitters."
Yost suggested that Duffy tends to blame himself too much for his on-field difficulties and finds it difficult to leave such things as Saturday's glitch behind him.
"He's got a tremendous desire to pitch here, he knows he can pitch here and he's doing everything he can to pitch, and when something like that happens, you blame yourself," Yost said. "But the thing that he's got to do is not blame himself -- that's just the way it is."
Joseph gave up just one hit along with three runs in his 5 2/3 innings. He struck out nine, but also walked six.
"[He lacks] command of his fastball, he'll continue to work on that," Yost said. "He's got a nasty slider, but when they know he can't command his fastball, they're just going to spit on that. Because if he gets a lot of swings and misses on bad pitches, they're just going to start taking it. Once he starts commanding his fastball, then he's got 'em."
Wade and Walters had high ERAs, 8.44 and 9.45, respectively, but they made a solid impression.
"Both guys, we liked a lot," Yost said. "They're both strike-throwers, and you had a real good idea what you'd get when you brought them in the game. They're going to throw strikes and compete."
The moves left the Royals with 33 active players in camp, with Rauch and Bueno looming as the last contenders to join Greg Holland, Tim Collins, Kelvin Herrera, Aaron Crow, Louis Coleman and Wade Davis in the bullpen.
At least Bueno and Rauch are the last visible contenders.
"There are a lot of things that can happen, let's put it that way," Yost said. "But we're at the time of spring where there's always waiver wires, there's always things that can happen. To pick your pitching staff five days before you leave doesn't make any sense, because you do it and then, bam, something pops up."
The Royals break camp on Wednesday, heading to Milwaukee for the final two Spring Training games on Friday and Saturday.