"It's like something burning," Perez said, describing the pain that he said has been bothering him for three to four starts.
Perez said Tuesday that he simply couldn't go on after the third inning, and he told Acta he needed to come out of the game. Jesus Colome came on in relief just before the first of two lengthy rain delays.
"I've been battling, but today, I couldn't hold it," Perez said. The southpaw added that a team doctor confirmed that it was only tendinitis, saying that he felt no discomfort while the doctor was running other tests on the arm and shoulder.
Acta first revealed the injury, saying it had not bothered Perez to the point where he couldn't pitch until tonight.
"Maybe three starts ago or something [it bothered him] ... but it wasn't anything serious that night," Acta said.
Perez has been one of the veteran anchors of the Nationals' pitching staff this year, though his numbers haven't been as impressive as some of his younger counterparts in the rotation. The Opening Night starter is 2-5 with a 4.09 ERA.
Perez and Acta both originally said the lefty would miss his next start. However, Perez backed off that comment later, saying he would see how his shoulder responds to treatment.
"I don't know," Perez said. "We'll see how it reacts on Thursday or Friday."
Zachary Osterman is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.