Jockey Ryan Fogelsonger, the nation's top apprentice rider last year, was taken off his mounts Saturday after being thrown at the start of Friday's final race at Hollywood Park.
Fogelsonger injured his right shoulder when Likely Prospect, the 7-10 favorite in a $25,000 claiming race for fillies and mares, stumbled two steps away from the starting gate and unseated her jockey.
Michelle Barsotti, jockey agent for Fogelsonger, said initial x-rays were negative, but that the jockey would seek further medical attention.
"He's in some pain," she said. "He's injured his shoulder before and he's just going to make sure everything is okay."
She said Fogelsonger could miss his mounts on Sunday as well.
Jockey Patrick Valenzuela missed his fourth consecutive day of calls when he called in with the flu Saturday.
With jockeys Julie Krone and David Nuesch also out of action following a two-horse spill earlier Friday, there were 21 late rider changes on Saturday's card.