The Royals placed Stratton on the 15-day injured list Saturday with a right forearm flexor strain.
Stratton surrendered two earned runs over an inning out of the bullpen Friday, but he appears to have come away from his latest outing with an injury. He won't be eligible to return from the IL until the final game of the season Sept. 29, and it's possible the Royals elect to keep him on the shelf until the start of the postseason, assuming they hang onto their wild-card spot. Until then, Angel Zerpa will come up from the minors to fill the opening in Kansas City's bullpen.