Casino Drive, at 7-2, is the Saturday race's second pick behind heavy favorite Big Brown.
Nobutaka Tada, racing manager for Casino Drive's owner, Hidetoshi Yamamoto, said a veterinarian examined the 3-year-old colt for a possible bruise, but no other details were released.
Tada said "something was slightly different" in the way the horse was walking on the racetrack grounds.
"At the moment we are not thinking of scratching," Tada said. "We don't know what's going to happen tomorrow."