After throwing a pitch to Washington's Matt Wieters, Ray called the Diamondbacks' trainer from the dugout and began motioning toward his right side. He exited the game after a brief conversation with the trainer and manager Torey Lovullo and was replaced by left-hander T.J. McFarland.
"I felt great up until that one pitch," Ray said. "I was probably in the middle of my delivery, I was letting go of the ball and just felt my side grab, and I wasn't able to finish the pitch and left it up and away. I just wasn't able to get anything on it."
Lovullo said Ray, 26, will undergo an MRI on Monday.
"The DL is likely," Lovullo said. "You guys obviously can assume that."
Ray, 26, is 2-0 with a 4.88 ERA in six starts this season with 45 strikeouts in just 27⅔ innings. He struck out two and allowed one hit on Sunday in his 1 1/3 innings.
Any long-term injury to Ray, a 2017 All-Star, would further weaken the rotation of the first-place Diamondbacks, who already will be without Taijuan Walker (elbow) for the remainder of the season.
Information from the Associated Press was used in this report.