Ruby Walsh was taken to hospital after a crunching fall from Corrick Bridge in the Rainbow Telecom Handicap Chase at Down Royal, with what appeared to be a leg injury.
The day had gone according to plan earlier for Walsh after Paul Nicholls' Kauto Star returned to action with a win in the JNwine.com Champion Chase.
In the very next race he partnered The Nightingale to success in the Grade Two Ladbrokes.com Chase, which meant he had won both of the big events on the card.
However, Walsh came crashing down on the first circuit on Tony Martin's Corrick Bridge at the fifth fence and was taken from the track in an ambulance.
Speaking at Dundalk, Walsh's sister Jennifer said the rider was waiting on X-rays and official confirmation, but that he thought his leg was broken.