Bobbyjo, winner of the 1999 Martell Grand National, will miss Aintree and faces a lengthy spell on the easy list after suffering an injury at Fairyhouse on Sunday.
The Tommy Carberry-trained 11-year-old sustained a fracture at the back of his near-fore knee when finishing eighth behind Northern Sound in the Grand National Trial Handicap.
The vets checked Bobbyjo over this morning and Carberry said: "He's probably going to get a screw put in the bone at the back of his knee.
"He'll probably go to the vets for the operation but I have to arrange it yet."
Bobbyjo was quoted at 33-1 for this year's Grand National but has now been ruled out of the April 7th contest.
Carberry added: "It'll mean he'll be off the course for the best part of a year. Hopefully he'll come back all right but you never know with this sort of injury."
Bobbyjo ended a 24-year wait for an Irish-trained Grand National winner when beating Blue Charm by 10 lengths at Aintree in 1999, ridden by the trainer's son Paul.
Sunday's race was the 48th of Bobbyjo's career, during which he has won eight races and earned £357,265 in prize money.
Last year's winner Papillon and 1997 scorer Lord Gyllene are left as the only past Grand National victors who could run in this season's contest.