Cubs' pitcher Mark Prior left Friday's game in the fourth inning after being hit in the right elbow by a line drive hit by Colorado's Brad Hawpe. X-rays on Prior's elbow were negative, Cubs trainer Mark O'Neal said, but ESPN learned later Friday an MRI revealed a "slight" fracture. The Cubs will continue to evaluate Prior's injury to determine how much time he will miss. Hawpe led off the fourth and hit Prior's first pitch back up the middle. After striking Prior, the ball deflected to third baseman Aramis Ramirez who caught it for the first out of the inning. Prior collapsed and rolled in pain on the mound. He was helped off the field by the Cubs training staff. The right-hander was replaced by reliever Todd Wellemeyer. O'Neal didn't want to speculate on the severity of the injury, or how much time the ace might miss. "That's a tough question," O'Neal said. "It's really going to be determined on how sore he is when he comes in and how much inflammation he has. We just have to wait until he comes in tomorrow [Saturday]. There's no way we can tell right now how long this is going to be."