Robbie McNamara was transferred to the Mater Hospital in Dublin after suffering a series of injuries in a fall at Wexford.
The jockey was due to partner last year's Cheltenham Gold Cup winner Lord Windermere in Saturday's Crabbie's Grand National at Aintree , but trainer Jim Culloty will have to find a new rider for his stable star.
McNamara, who rode a double at last year's Cheltenham Festival as an amateur before turning professional earlier this season, was partnering 14-1 shot Bursledon when he came to grief four flights from the finish in the first division of the Cahore Point Handicap Hurdle.
He was taken to Wexford General Hosptial, where he underwent a CT scan, before being transferred.
Dr Adrian McGoldrick, chief medical advisor for the Irish Turf Club, said: "Robbie has chest, abdominal and spinal injuries and has been transferred to Mater (Hospital).
"I have spoken to his family and we have agreed there will be no further comment until Sunday, by which time he will have undergone further scans."
Lord Windermere, who was pulled up in this year's Gold Cup, is a general 50-1 chance for Aintree glory.
Culloty has a second runner in the shape of Spring Heeled, who is due to be ridden by Nick Scholfield and is available at 25-1. Also owned by Dr Ronan Lambe, he was last seen finishing fourth in the Bobbyjo Chase at Fairyhouse in February.