Cal. Code Regs. tit. 4, § 1545 – Substitution of Jockey or Trainer.

(a) In the event a jockey who is named to ride on a mount in a race is unable to fulfill his engagement and is excused by the stewards, the trainer of the horse may select a substitute jockey or, if no satisfactory substitute jockey is available, may elect that the stewards declare the horse from the race.
(b) In the absence of the trainer of the horse, the stewards may place the horse in the temporary charge of a substitute trainer of their selection.