Gray said Thursday he was told he won't be able to pitch in a game for at least a couple of weeks.
"It's really frustrating because I have so much stuff I need to work on, reps I don't need to miss out on," Gray said. "It could be a lot worse. I'm happy it's not bad. It's just a few weeks I'll be out. But it's frustrating because I needed these last couple games to get some reps in for the season."
The hard-throwing prospect exited Tuesday's game against Milwaukee after just 16 pitches. He said he felt sharp pain when he threw his changeup and curveball, and feared the worst.
"I was thinking maybe something was torn," Gray said, with an electric stimulation machine attached to his side. "I'd never felt anything like this before. I was kind of freaking out, so when they told me it was just a strain I was kind of relieved."
Gray, who has a 3.86 ERA in five uneven spring training appearances, said he first started feeling pain a few days ago but pitched through it. He felt better until he aggravated the area while warming up before Tuesday's start.
"He's had a couple tough outings," manager Walt Weiss said. "But he has been working on things. He's been willing to work on some things. I really liked the way. I really liked the way the ball was coming out the other day in a small glimpse, obviously."
Gray is to undergo heat and massage treatment on the muscle near the center of his waist.
The 24-year-old was the third overall pick in the 2013 amateur draft. He made his major league debut late last season and was expected to begin the season in the back end of a rotation that likely includes Jorge De La Rosa, Chad Bettis, Tyler Chatwood and Jordan Lyles.
Another rotation candidate, lefty Chris Rusin, hasn't pitched in an exhibition game yet due to a finger injury.