Acta said Pena would receive a shot of dye in the shoulder in advance of the examination, though he didn't know when to expect Pena back.
"It all depends on the decision of the doctor," Acta said.
Asked if he knew when the injury occurred, Acta characterized it as "old stuff" that has been bothering Pena for awhile, not just the last week or two. He added that Pena had a cortisone shot in the shoulder "about a month or so ago."
"He has never made excuses or anything," Acta said. "It just bothers him here and there, it's not because he can't play through it. But we need to take care of it, because it's bothering him."
Though Pena declined to comment on the injury, Acta said he understands why his power-hitting left fielder played through his pain so for long. He said all players compete with some kind of pain through the year, and he commended Pena for fighting through his.
"Nobody's 100 percent every single day," Acta said. "We appreciate [when] people play through pain, and I'm sure that if he wasn't able to take this pain and play through it, he wouldn't play through it."
Pena, who was hitting just .205 with two home runs entering action Sunday, has been working all season in the batting cage to improve his swing. However, Acta said he and the other coaches have tried to keep Pena positive and "minimize his extra hitting" so he doesn't overthink things.
"Guys want to fight through slumps and get better by extra working," Acta said. "Right now, I think it's a combination [of injuries and his slump] at the plate."
Zachary Osterman is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.