Davy Russell will miss the ride aboard Sir Des Champs in the Betfred Cheltenham Gold Cup after suffering a punctured lung.
The rider took a crashing fall from Un Beau Matin at the final flight in Wednesday's Coral Cup, but was fit enough to take up his first two mounts on Thursday's card.
However, after finishing unplaced on both Argocat in the Jewson Novices' Chase and Stonemaster in the Pertemps Final, Russell was ruled out for the rest of the meeting.
Bryan Cooper assumed his mounts aboard First Lieutenant in the Ryanair Chase and Bog Warrior in the Ladbrokes World Hurdle, but Russell's absence leaves trainer Willie Mullins and owners Gigginstown House Stud with a problem ahead of Friday's feature event.
Gigginstown manager Eddie O'Leary said: "Davy has a problem with his lung and he's down for the week now. I'm not sure (who will ride Sir Des Champs), we'll have a good chat with Willie about it first."
Michael O'Leary, owner of Gigginstown House Stud, confirmed that Russell had a punctured lung.
He said: "He has a punctured lung, which he's had to have drained. He's been stood down for today and tomorrow, he might need a small operation. It's quite serious and he can't travel home. His health is my biggest concern.
"I don't know who will ride Sir Des Champs, we'll talk to Willie overnight and decide who is the best available."
The track confirmed Russell had been taken to Cheltenham General Hospital for further treatment.
A tweet from Cheltenham read: "Davy Russell has suffered a spontaneous punctured lung. He has been treated at the course and is off to hospital. He is being accompanied by our senior doctor and is expected to stay in overnight for observation. Our best wishes to Davy."