With that in mind, Herrera was optioned to Triple-A Omaha and Louis Coleman, also a right-hander, was recalled from the Storm Chasers to take his place on Thursday. Herrera was the first player sent to the Minors by the Royals this season.
No other player moves are in the immediate offing in the aftermath of the Royals' recent plunge, according to Yost.
"There are a lot of things we're looking at and a lot of things we're thinking about, but nothing is on the horizon," Yost said.
The hard-throwing Herrera had a 2-4 record, two saves and a 4.87 ERA in 19 games. His primary problem was giving up home runs -- there were eight of them among the 19 hits he surrendered this season.
This was an abrupt change from Herrera's rookie season of 2012 when he posted a 2.35 ERA with a 4-3 record and three saves in 76 games. He gave up just four homers in 84 1/3 innings.
Coleman had a 2-1 record, three saves and a 1.03 ERA in 14 games for Omaha. He's given up just three runs in 26 1/3 innings. He spent parts of the last two seasons with the Royals, appearing in a total of 90 games with a 3.25 ERA, a 1-4 record and one save.
"With Kelvin Herrera, it had gotten to the point, I think, where he lost some confidence in his fastball," Yost said. "What we're looking to do with him is get him back down. He needs to incorporate his secondary pitches -- which are both plus pitches, his curveball and his changeup -- into his repertoire, adjust his mechanics a little bit. He got to the point by not having confidence in his fastball to where he was trying to overthrow it, so he needs to just smooth his mechanics a little bit and really just go down and have some success."
Surrendering so many home runs became a problem in Yost's view. The manager noticed a change in a win over the Angels on May 15, when Herrera gave up a home run, a double and two walks in two innings.
"It really started in Anaheim. He was overcompensating, trying to overthrow the ball because he'd lost confidence in his fastball, which resulted in horrible mechanics for that day," Yost said. "We geared him down, got his mechanics back in line, but I think in the back of his mind was that thought of giving up a home run every time he threw a fastball was there.
"It's mostly a confidence thing. We want to send him down for a period of time, let him have some success, get some confidence back in his abilities and his fastball. Incorporate more use of his good curveball, his good changeup and then bring him back."
Coleman learned about his promotion while having coffee on Thursday morning in Omaha. He got to Kansas City just in time to join the Royals for batting practice before their game against the Angels.
There's no mystery behind his outstanding record at Triple-A, which included no runs in his last 14 1/3 innings.
"Just throwing strikes and not getting behind hitters," Coleman said. "Last time I pitched I had a line-drive out that could've turned into a double. It's just little things like that are kind of going my way right now. I'm sure it'll catch up to me, but hopefully not any time soon."
Coleman was in the Royals' Spring Training camp but didn't make the club despite a 1.46 ERA and a 2-0 record. So he's a known commodity and also knows his way around.
"Just being able to come into the clubhouse and knowing all the guys, shaking hands and being on a first-name basis just makes it a little easier transition," Coleman said. "But the hitters are still really good and that's going to make it tough."
His 2013 debut was a success. He pitched a scoreless ninth inning in a 5-4 loss to the Angels, working around a two-out walk.
Getting sent down is a new experience for Herrera, who shot up through the system -- from Class A to the Majors -- in one season, 2011, after recovering from a stress fracture in his elbow. So, too, is the experience of not succeeding.
"Never. That's what he said: 'I've never been through this,' " Yost said.
Herrera's most pressing need is to improve the location of his pitches, Yost said.
"He's very young, too, and a big part of our pen so we need to get him straightened out," Yost said of the 23-year-old. "Get a little bit of his swagger back and bring him back."
Dick Kaegel is a reporter and Kathleen Gier is an associate reporter for MLB.com.