The Reds placed Johnson on the 15-day injured list Saturday with right elbow inflammation, retroactive to Wednesday.
Johnson's injury helps to explain why he hasn't pitched in a game since Tuesday, and it will also keep him out of action for at least another couple of weeks. Tony Santillan figures to handle a larger share of high-leverage opportunities while Johnson is out, and Lyon Richardson will come up from Triple-A Louisville to fill the void in Cincinnati's bullpen.