"I felt like something popped," Pena said. "Something was completely different. Sometimes, I've felt I've had hamstring [issues] and played through it. I didn't want to hurt my team. I felt like something popped and we were building something good in that inning. It was a little painful."
Pena was lifted for pinch-runner Ryan LaMarre, who later scored to make it a one-run game. Since Tucker Barnhart was lifted for a pinch-hitter just ahead of Pena in the eighth, third catcher Ramon Cabrera took over behind the plate.
Initial tests by medical director Dr. Tim Kremchek left Pena hopeful that it might not be a serious injury.
"He felt like it was going to be a couple of more days, but we'll know tomorrow much better," said Pena, who is batting .282 in 95 games this season.
Mark Sheldon is a reporter for MLB.com. Read his blog, Mark My Word, follow him on Twitter @m_sheldon and Facebook and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.