"I believe that when I threw a changeup to Alvarez, I felt something," Cueto said. "I felt something sharp in my arm."
The Reds removed Cueto in a precautionary move.
"I feel worried because I was feeling really well," Cueto said. "I didn't want to come out of the game. I was pitching so well that I really wanted to stay in that game tonight."
Cueto tossed 4 1/3 innings and surrendered one run on two hits. He threw 52 pitches, 35 for strikes.
Right-hander Alfredo Simon came on in relief of Cueto and allowed a run before escaping the inning. He later gave up another run, taking a loss in the 3-1 game.
The Reds are hoping for the best, as Cueto is 1-0 with a 2.60 ERA.
"We don't know much yet," manager Dusty Baker said. "It's not anything in the elbow and not anything in the shoulder. It's in the belly of the triceps."
George Von Benko is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.