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.
"Right up to the end, he was trying to impersonate a four-year-old," said Mullins on Friday. "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.