Mercer limped slightly after standing up. Soon after, Pedro Florimon entered as a pinch-runner.
"I slid over the bag, because I didn't realize how fast I was, I guess," Mercer said with a smile.
Mercer, 29, said he left the game for precautionary reasons and that he should be fine to return Monday.
"The front of my knee impacted the front of the bag," he said. "It was nothing new. Just precautionary."
If Mercer's situation isn't serious, that would be a welcome development for the banged-up Pirates. Reliever Jared Hughes (left lat), outfielder Starling Marte (right quad tightness), outfielder Gregory Polanco (right shoulder discomfort) and first baseman/outfielder Michael Morse (leg) are all recovering from injuries.
Pirates manager Clint Hurdle is optimistic about Mercer's condition.
"I think Jordy is going to be fine," Hurdle. "He jammed his knee into the base when he slid, so I think it's a small contusion."
Andrew Astleford is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.