Mikey Fogarty will miss the ride on Vesper Bell in the Crabbie's Grand National at Aintree on Saturday after fracturing two vertebra in his back in a fall on Friday.
The jockey could be sidelined for up to 12 weeks following his tumble from Bishopsfurze in the Crabbie's Supporting Everton In The Community Topham Chase over the famous fences on Friday.
"He's got two fractured vertebra in his back. He's had a scan and will have a further scan tomorrow morning to decide whether he needs surgery," said the Irish Turf Club's senior medical officer, Dr Adrian McGoldrick.
"He's going to be out probably for between eight to 12 weeks. That's what you would normally expect for that kind of injury."
Paul Carberry still hopes to be fit to partner Monbeg Dude for Michael Scudamore after he was unseated from Kauto Stone at the eighth fence.
Like Fogarty, Carberry was taken to the University Hospital, Fazakerley, for precautionary X-rays for a possible pelvic injury but his results revealed no damage.
Two other jockeys with rides in the National, Andrew Tinkler and Noel Fehily, took knocks in the same race.
Andrew Tinkler banged a wrist after Tanks For That fell at the 12th while Fehily had a problem with an elbow after he came down at the 15th on Fago. They missed their remaining rides.
Carberry, Tinkler and Fehily will have to be passed fit by the racecourse doctor on Saturday if they are to take up their National rides.
Sean Quinlan was another to be hurt on the day when parting company with Party Rock in the two-and-a-half-mile handicap hurdle.
After being treated on the track he was also taken to Fazakerley for further assessment, but was discharged earlier this evening.