Left-hander Danny Duffy, who has been recovering from Tommy John surgery, was reinstated from the 60-day disabled list and optioned to Omaha. Left-hander Francisley Bueno was designated for assignment to clear roster space.
Smith allowed four earned runs in four innings in his only appearance with the Royals earlier this season, but he was 5-3 with a 2.88 ERA at Omaha.
Herrera, who's just 3-5 with a 5.20 ERA, is headed to Omaha for the second time this season.
The Royals also announced that outfielder Quintin Berry, who was designated for assignment Sunday, had accepted an outright assignment to Omaha.