Jansen, 6-foot-4 and 220 pounds, was doing a pitcher's fielding practice drill requiring him to field a slow roller between the mound and first base. Jansen tried to glove the ball and flip it in one motion when his left leg went out from under him and he landed on his right jaw.
"I feel weak and dizzy," Jansen said after returning to the clubhouse. "I don't know what happened. My right hand was in the air and my glove was going toward the base and I hit my jaw and scraped my left knee."
Jansen, who pitched a scoreless inning Thursday night, felt better after resting and watched five innings of the game from the dugout.
In other injury news, Marcus Thames participated in outfield drills and served as designated hitter in Friday's game with the Giants as his heel, irritated by new spikes, was improved, according to manager Don Mattingly. Thames said he has had plantar fasciitis in the past and believes he caught it this time before it became a serious problem.