WILLIE Mullins described Un De Sceaux as a "once-in-a-lifetime horse" as he confirmed the multiple Grade One-winner's retirement following injury.
The career-ending problem was discovered after the 12-year-old worked on Thursday morning.
Un De Sceaux finished outside the first two just once in 21 completed runs over fences - the majority of which were in Grade One company.
Owned by the O'Connell family in East Cork, Un De Sceaux had a huge following on Leeside and beyond.
"Right up to the end, he was trying to impersonate a four-year-old," said Mullins.
"He worked great yesterday morning, but you could see on pulling up that he wasn't right, and it was confirmed this morning that he had a ligament injury."
Un De Sceaux has been among the brightest stars at Mullins' powerhouse stable since arriving from France seven years ago.
The champion Irish trainer added: "He's been a huge star in our yard and he'll be hugely missed.
"You can't replace horses like him - it's impossible. He was a once-in-a-lifetime horse, and we were very lucky to have him.
"It was incredible how sound he was, because he was so hard on himself, and every day he gave his best in racing and in training.
"He had a huge following. The owners (the O'Connell family) have Bachasson to go to the (Cheltenham) Festival with this year in the Coral Cup, and hopefully they can look forward to that."
Un De Sceaux ends his career having won 23 of 34 races under rules and more than £1,500,000 in prize money.
He ran at five Cheltenham Festivals, winning the Arkle in 2015 and the Ryanair in 2017.
Owner Edward O'Connell's son, Colm, said: "When I got the call from Willie I had to pull the car over.
"It was always going to happen - this day was always going to come - but we'd planned to go to Cheltenham and retire him on the spot. But then you never know when to do it as if he was beaten a length we might have been tempted to go to Punchestown.
"He'll go back to France in a couple of months. Ginnie (Virginia Bascop) who looks after him has a plan to take him back there. You don't put Eric Cantona in a nursing home.
"No one has died, it's just that when he's been around for so long - not just a year or two - it's very hard to take.
"This would have been his sixth consecutive Cheltenham Festival. He won the Arkle, ran in the Queen Mother, won the Ryanair and has been in another two Ryanairs.
"He was going to run in the Queen Mother this year and Patrick (Mullins) was going to ride him. We quite fancied him to run really well given the ground.
"Whenever he ran we took over 30 people from Cork with us, it was like a syndicate. We certainly enjoyed it, we knew we'd never get another like him so we made sure of that.
"I don't think Willie has ever had a horse like him."