Ruby Walsh's Cheltenham Festival came to a premature end after a crashing fall in the first race left him nursing a broken arm.
The rider was diagnosed with a compound fracture of his right humerus, the bone in the upper arm, after parting company with Abbyssial in the JCB Triumph Hurdle.
Walsh was leading the field over the second hurdle when taking a crunching spill, bringing down stablemate Adriana Des Mottes in the process.
Walsh was able to walk gingerly to the ambulance, but could be seen holding his arm.
The rider was set to undergo surgery on the open fracture at Gloucester Hospital.
A racecourse spokesperson said: "He has a compound fracture of his right humerus. He has been taken to Gloucester for surgery."
Walsh's understudy at the Willie Mullins yard, Paul Townend, the rider of Adriana Des Mottes, rode in the second race but was then also stood down for the day, with David Casey taking the rides on Briar Hill and On His Own in the Gold Cup
Mullins said of Townend: "He wasn't feeling too well and they decided the best thing to do wasn't to ride."