Seabass has been withdrawn from the Crabbie's Grand National after suffering a serious injury at Down Royal on Monday.
Trained by Ted Walsh and ridden by his daughter Katie, Seabass was third at Aintree in 2012 and went off the 11-2 favourite last year, only to finish 13th.
However, the 11-year-old finished last in his warm-up in the Daily Mirror Chase, after which he was found to have sustained ligament damage.
Pat Glynn, of owners the Gunners Syndicate, said: "He'll be out of action for at least six months and maybe eight (months).
"We were looking forward to him at Aintree again as he was coming back to form.
"He suffered ligament damage mid-way through the race. We thought he wasn't 100 per cent, and when he got home he was very lame. He had a scan this morning and they found ligament damage.
"He's had tendon trouble before and made a full recovery. If he's OK we might put him back into training, but we're not sure about that at the moment.
"He'd been working very well at home and we had big hopes for him. He's been a very good servant for us."
Tidal Bay remains the top weight of the 73 horses still engaged on April 5 after 27 came out at the latest forfeit stage.
Those to have been taken out include several others owned by Graham Wylie such as last week's controversial Cheltenham Gold Cup runner-up On His Own.
Others to have been scratched include Kauto Stone, Cantlow, Carlingford Lough, Cape Tribulation, Mon Parrain and Pandorama.