Barry Geraghty will miss the ride on leading Crabbie's Grand National candidate The Druids Nephew after suffering a hairline fracture to his left tibia in a fall at Downpatrick on Sunday.
The leading jockey was riding Sailors Warn for Edward O'Grady in the Visit toals.co.uk Rated Hurdle and still held every chance when he fell at the second-last.
While Geraghty did not take up his ride in the following race, his injury was not thought to be serious, with the rider believing he had only bruised his shin.
However, later in the afternoon Geraghty went to hospital in Blanchardstown, Dublin, for an X-ray which confirmed the bad news.
Dr Adrian McGoldrick, chief medical officer for the Irish Turf Club, said: "Barry has a hairline fracture of his left tibia.
"I've just left him, his leg is in plaster and he's gone home.
"He'll see a specialist on Tuesday and then he'll have an idea about how long he'll be out but I would imagine it will be between four to eight weeks."