Irish jockey Bryan Cooper suffered a broken leg in a crashing fall at Cheltenham on Tuesday.
Cooper, who is retained rider for powerful Irish owners Gigginstown House Stud, fell two out aboard the Gordon Elliott-trained Clarcam in the Fred Winter Juvenile Handicap Hurdle.
The highly-rated jockey had been due to partner Last Instalment in the Betfred Cheltenham Gold Cup on Friday.
Gigginstown racing manager Eddie O'Leary said: "He's broken two bones in one of his legs."
Cooper only returned to action in September after he suffered a similar following a fall at Down Royal in May.
The 21-year-old jockey shot to prominence last season when he secured three winners at the Cheltenham Festival.
He then added two further Grade One triumphs a month later at Aintree.
