Popular chaser Monet's Garden has been retired and "is battling away for his life", said trainer Nicky Richards.
The bold-jumping grey, who turned 13 on New Year's Day, is critically ill after suffering an infection in a foot.
He was last seen winning the Old Roan Chase for the third time at Aintree in October, but it was reported after the race that he had a problem with a foot.
The injury became infected in the navicular bone area, which required several operations, but the wound has taken far longer than anticipated to heal. Richards said: "The old horse is battling away for his life.
"I spoke to our vet this morning. It has been a hell of a job trying to close this infection down.
"The navicular bone became infected and it's into the flexor tendon as well and the infection is taking some closing down. The lady looking after him has done a fantastic job.
"She really has tried her best. He's had the best treatment available.
"Regarding his racing career though, that's over. We are just trying to save the old horse now, he's still at the vets.
"I don't know if he's over the worst yet, even. We are having a hell of a job closing this infection down. If we can do it, it will be absolutely fantastic."
Michael Hourigan will for the time being play it safe with his useful- looking prospect Mr Cracker.
A winner of three of his four starts over regulation fences, the six-year-old chestnut was particularly impressive on his most recent start at Limerick a few days ago when he ran his three rivals into the ground for a distance victory in the Grade Two Greenmount Park Novice Chase.
Mr Cracker is entered in the Frank Ward Solicitors Arkle Novice Chase at Leopardstown on January 23, while Hourigan mentioned after Limerick his main target could be the Powers Gold Cup at Fairyhouse.
"We want to look after him this season, and there's no real plan at the moment," said Hourigan.
Big Buck's will not be seen on the track again until he bids for a third consecutive Ladbrokes World Hurdle at the Cheltenham Festival.
Trainer Paul Nicholls had considered slotting in another start at Prestbury Park after he took his winning sequence over timber to 10, following another laughably easy success in the rearranged Long Walk at Newbury.
However, Nicholls reported: "I have had a discussion with (owner) Andy Stewart this morning and we have decided that Big Buck's will sidestep the Cleeve Hurdle and go straight to the Cheltenham Festival.
"Taking in the Cleeve Hurdle was our original plan.
"But he will be given a break instead and his campaign will now mirror last season's, with the Cheltenham and Aintree Festivals as his next targets."