To take Kelley's place on the roster, the club recalled left-hander Garrett Olson from Triple-A Tacoma, where he was 2-2 with a 3.08 ERA in five starts. Olson joined the club in time for Wednesday night's game in Kansas City.
After seeing a report from the team doctor, Mariners manager Don Wakamatsu said Kelley will likely be out longer than 15 days.
"A month-to-six weeks right now, probably," Wakamatsu said before the game against the Royals. "Hopefully, it's a month."
Kelley has been a key figure in the American League West leaders' bullpen, going 1-1 with an ERA of 1.54 in 10 appearances, nine of them scoreless.
"It's not real good. He tore it pretty good," Wakamatsu said. "It's an ab strain. It's going to take some time to heal. We hope he gets back sooner than they say it is. But he strained it pretty good."
Acquired along with Ronny Cedeno in exchange for Aaron Heilman in a January deal with the Cubs, Olson spent most of last season in the Orioles' rotation. He was 9-10 while ranking fourth in starts (26), fifth in wins and innings pitched (132 2/3) and sixth in strikeouts (83) among American League rookies.
"Our thought is just to have some protection length-wise," Wakamatsu said.
Olsen was supposed to start for Triple-A Tacoma on Tuesday but was scratched from that start to be ready for the call up.
"If we need some length early or late, similarly what we did with [Jason] Vargas the other day," Wakamatsu said. "It might allow me to bring in Vargas in a lefty-lefty situation."
Ruston Dodd is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.