I think it very much depends on what part of the body a horse gets kicked in as to how much it can affect them but once the adrenaline of racing kicks in it can disguise a potential issue until it wears off.
Henbit won the 1980 Derby and afterwards it was discovered that he had a hairline fracture in his canon bone…….horse are remarkable creatures and as we have seen can run through pain barriers to the extent where they have sadly collapsed and died soon after a race.