Park suffered a right hamstring strain in the seventh inning of Wednesday's 6-1 victory over the Nationals and did not return.
"When he threw the ball, like I see him, he almost went down," Phillies manager Charlie Manuel said. "He pulled a hammy, and he pulled it pretty bad."
The 36-year-old righty was in visible discomfort in the clubhouse afterwards, walking with an obvious limp and struggling to put on his jeans. He said he felt significant pain.
Park hurt his push-off leg while throwing the 10th and final pitch of his outing. He induced a soft line drive from Ryan Zimmerman to first base, which turned into an inning-ending double play. The barrel of Zimmerman's bat came splintering towards Park, but that had nothing to do with the injury, which happened during the delivery.
"This is like a pop," said Park, who will be reevaluated by head trainer Scott Sheridan and the team's medical staff on Thursday.
"Scott said usually if it's really bad, you can't even walk," Park said. "So I can walk around here. Tomorrow could be a little bit better than today. But sometimes it can get worse than that -- that's negative."
After losing his rotation spot in May, Park has been stellar out of the bullpen. The 36-year-old veteran has allowed 14 earned runs in 50 relief innings (2.52 ERA), picking up 15 holds, striking out 52 and walking just 16. He has earned the trust of Manuel and become a stabilizing presence while other relievers went down with injuries and struggled with inconsistency.
David Gurian-Peck is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.