"I felt a pop," he said.
Williams had allowed four runs to score in the first inning when he uncorked a wild pitch to David Lough, which went toward the Phillies' dugout. The ricochet allowed two runners to score on the play, which included Williams unintentionally doing a split while trying to field a throw at the plate from Carlos Ruiz.
Williams grimaced in pain as he fell to the ground.
"I just rolled over and didn't move," he said.
Williams has a 6.43 ERA, which is the second-highest ERA among 107 qualified pitchers in baseball.
"I felt like I was going to turn the corner and then something like this happens," Williams said. "Can't dwell on it though. Just have to get better."
Todd Zolecki is a reporter for MLB.com. Read his Phillies blog The Zo Zone, follow him on Twitter and listen to his podcast This story was not subject to the approval of Major League Baseball or its clubs.