Barry Geraghty is set to miss the Aintree Grand National on joint-third favourite The Druids Nephew after suffering a hairline leg fracture.
Geraghty was riding Sailors Warn for Edward O'Grady at Downpatrick on Sunday when he fell at the second last.
The 35-year-old originally believed he had only bruised his shin but he is now set to be out for four to eight weeks.
"Barry has a hairline fracture of his left tibia," said Irish Turf Club chief medical officer Dr Adrian McGoldrick.
"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."
The Druids Nephew is currently about 14-1 joint third favourite behind 8-1 chance Shutthefrontdoor, the potential mount of retiring champion jockey AP McCoy in his final Grand National.
Geraghty is the third top jockey to be ruled out of the National, joining injured Davy Russell and suspended Bryan Cooper on the sidelines.
The 2015 Aintree Grand National Festival will take place from 9 to 11 April.
Geraghty won the 2003 Grand National on Monty's Pass.