KANSAS CITY -- Right-hander Kelvin Herrera, the Royals' hard-throwing but inconsistent reliever, was optioned to Triple-A Omaha on Tuesday and left-hander Will Smith was recalled from the Storm Chasers.

Herrera departs to Omaha for the second time this season with a 3-5 record, a 5.20 ERA and two saves. His latest glitch came in Sunday's 7-6 victory when he gave up a walk and a two-run double to the White Sox in the seventh inning.

"He's just struggling right now," Royals manager Ned Yost said. "Struggling with mechanics, struggling with command. We need to get him straightened out."

In another move, left-hander Danny Duffy was removed from the 60-day disabled list, reinstated and optioned to Omaha as his Minor League injury rehabilitation time expired. To make room on the 40-man roster, left-handed reliever Francisley Bueno was designated for assignment. Bueno was 1-2 with a 3.40 ERA in 23 games for Omaha.

Yost said that Herrera would be used as a starter with Omaha, but not with the intention of making him starter.

"It's just so we can get him back to using all of his pitches," Yost said. "It's everything -- his controlling the running game got away from him. It's just [a matter of him] slowing the game down again and being able to execute."

It's been a tough second year for Herrera. As a rookie in 2012, he worked in 76 games with a 2.35 ERA, a 4-3 record and three saves. This year, he was counted on as the eighth-inning setup man, but couldn't stay in the groove. Herrera was sent to Omaha on May 23 for a 10-day stay and, in five games, pitched 4 2/3 scoreless innings with two hits, three walks and six strikeouts.

For Kansas City, Herrera has given up eight home runs, twice as many as all of last season, and 15 walks in just 27 2/3 innings.

"I think it's just confidence a little bit. He's given up the homers and is a little shell-shocked," Yost said. "Then, he started fighting it, started to overthrow and lost his mechanics. It's kind of a spiral effect."

Herrera has frequently been touted as a closer of the future. His fastball sometime reaches 100 mph or more but, too often this year, his control has been off.

"It's about command, being able to throw your pitches to a quality spot in the strike zone instead of just rarin' back and firing it. [If you're] behind in the count and throw one over the middle, as we've seen, they'll whack it," Yost said. "And he needs to incorporate more of his secondary pitches -- just, in effect, become more of a pitcher than a thrower. And he combined those two aspects last year."

Smith joins the Kansas City roster for the third time this season. He was called up twice as the 26th player for doubleheaders. He started the second game against Cleveland on April 28 and was the loser in a 10-3 Indians victory.

Recently, in anticipation of a possible promotion, Smith was switched from starting to the Omaha bullpen. In seven relief outings, he had a 0.84 ERA and one save. His overall record was 5-3 with a 2.88 ERA in 17 games.

"Everybody that saw him was pretty impressed," Yost said. "The quality of his stuff has elevated since he's gone into the 'pen. So, we'll see."

Smith gives Yost a third lefty in the bullpen, along with Tim Collins and Bruce Chen. The eighth-inning job, Herrera's until recently, will be assumed by Aaron Crow and Collins.

Bueno had a 1.56 ERA in 18 relief appearances last year for Kansas City, but did not make the club in Spring Training. For Omaha this year, Bueno had a 3.40 ERA in 23 games, but there were other lefty candidates with the Storm Chasers.

"[Everett] Teaford's been throwing good, we have Will Smith, Donnie Joseph and Buddy Baumann's elevated himself into the picture now with his production," Yost said. "So, we had some options there."

The Royals also announced that outfielder Quintin Berry had agreed to an outright assignment with Omaha. He'd been designated for assignment last weekend to make room on the 40-man roster for pitcher Maikel Cleto. Berry was claimed off waivers on June 4 from Detroit and was hitting .194 for Omaha.