The Royals shipped the hard-throwing reliever to Triple-A Omaha for the second time this season on Tuesday, and recalled left-hander Will Smith to provide depth in their bullpen.
Herrera was considered one of the bright young pitchers in the American League last season, when he blew away hitters with a fastball that nudged past 100 mph. But he has been prone to giving up home runs this season, and was 3-5 with a 5.20 ERA in 29 appearances.
By contrast, Herrera was 4-3 with a 2.35 ERA in 76 relief appearances last season.
"It's an issue where I think he gave up some home runs and started to try to throw the ball harder and lost mechanics and just got out of whack," Royals manager Ned Yost said before Tuesday night's series opener against the Atlanta Braves.
"He's going to start down there," Yost said, "not with the intention of making him a starter, but at least get him on a rotation where he knows when he's pitching. ... We're just trying to get his mechanics back in line, try to get him to slow down the game a bit."
Smith made one start for the Royals earlier this season, allowing four earned runs over four innings against Cleveland. But he has been much better at Omaha, where he was 5-3 with a 2.88 ERA working in both the rotation and out of the bullpen.
"He's been throwing the ball extremely well down there," said Yost, who plans to use Smith against right- and left-handed hitters. "It gives us a little depth until Kelvin gets going."
Along with swapping relievers, the Royals activated left-hander Danny Duffy from the disabled list and optioned him to Omaha. Duffy had an 11.70 ERA in three rehab outings for the same club as he continues to recover from Tommy John surgery.
The Royals created space for him by designating left-hander Francisley Bueno for assignment.
Felipe Paulino, who is also coming off Tommy John surgery, took the mound at Kauffman Stadium for a side session Tuesday and felt better after shutting down his rehab due to a back issue.
He hopes to make a minor league start later this week.
"The arm feels good," Paulino said. "I'm going here for one last training (session), and like I say, it's nice and easy today and we'll see what's going on this weekend."