The Royals placed Harvey on the 15-day injured list Saturday with mid-back tightness.
Manager Matt Quatraro revealed after Friday's game that back tightness is the reason Harvey hasn't made an appearance since Sunday, and the 29-year-old righty will now officially be sidelined until at least Aug. 23. Lucas Erceg and James McArthur figure to be the leading candidates for saves while Harvey is out, and the Royals will bring up Carlos Hernandez from Triple-A Omaha to replenish their bullpen depth.