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Jockey Kerwin John has taken the mount on Concerto's Thunder in Monday's seventh race, replacing Felipe Valdez, who took off his final two mounts of the day.
Track stewards said they were told by first aid staff that Valdez's back had "flared up."
Valdez's absence led to the withdrawal of 20-1 shot Tribal Nation in the fourth race, when trainer Anthony Saavedra could not find a suitable rider and was turned down by a few other jockeys.
