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.
Russell has ridden Sir Des Champs on his last six outings, including his Irish Hennessy win.
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 has a punctured lung which he's had to have drained," said O'Leary.
"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."
Dr Adrian McGoldrick, chief medical officer of the Irish Turf Club, later issued an upbeat bulletin. "Davy is in good spirits and in good shape," he said.
"He will be out on Saturday when we can get him home and he can then begin his recovery."