The Mariners ended up losing Tuesday's game, 7-2, in 10 innings. Worse yet, they figure to lose Kelley's services for several days, if not weeks.
Kelley, one of the biggest surprises in Spring Training, entered Tuesday afternoon's series finale against the Rangers in the top of the 10th inning with a 1.54 ERA.
After the second pitch he threw Texas shortstop Elvis Andrus, Kelley walked off the mound and raised his left arm, as if to stretch his left side.
He climbed back on the mound, threw another pitch, and fell to the ground.
"When he first stepped off the mound, my first thought was, 'Is he all right?'" catcher Rob Johnson said. "I thought he was just stepping off to catch his breath, or maybe he tweaked something.
"But after the next pitch he threw, you knew something was bad. It was scary and frustrating at the same time. He's a big guy for us, and we need him. And then you are scared when you see something like that."
Manager Don Wakamatsu has experienced enough torso injuries to know that when Kelley went down and grabbed his side, this could be a serious injury.
"He was in a lot of pain, and the first thing you do is pray that it's not too severe," the manager said. "But I have seen too many guys grab their side like that. You know it's going to take some time to heal."
Kelley was under medication after Tuesday's game and unavailable to the media.
But his loss for any length of time is a tough blow to one of the most surprising teams in the Major Leagues this season.
"He was pitching so well," Wakamatsu said. "He has done just a phenomenal job and kept climbing the ladder. We had even been talking about him closing some games. This is pretty devastating."
The Mariners are, however, expected to get No. 1 closer Brandon Morrow back on Saturday, the first day he's eligible to come off the 15-day disabled list. Morrow has been shut down with right biceps tendinitis.