JOSEPH O'Brien said he was indebted to St Nicholas Abbey, who died yesterday, for being the fulcrum of his emerging riding career.
The Aidan O'Brien-trained seven-year-old, winner of six races at the highest level, including three Coronation Cups at Epsom, had to be put down after he suffered another bout of colic.
O'Brien jnr rode him to two Coronation Cup triumphs and also secured notable victories on the worldwide stage in the 2011 Breeders' Cup Turf, where he became the youngest winning rider in the meeting's history, and in the 2013 Dubai Sheema Classic at Meydan.
Reflecting on his Breeders' Turf success, the jockey said: "He will always be very special to me, and so will that day.
"It was my first big win on the world stage, and it was my first big win outside Ireland. He's a horse I will never forget.
"I think his best win was in the Sheema Classic in Dubai, but he was very good in his third Coronation Cup, as well.
"In his last season, he was so much better than he had ever been. He was so relaxed. He loved his racing and he loved his work.
"It is sad to see him go that way, we would all have liked to have seen him go to stud. He has given us all here at Ballydoyle some great days."
St Nicholas Abbey's third Coronation triumph last June – his final racecourse appearance – took his career earnings to almost £5million.
A statement issued by Coolmore read: "Regretfully St Nicholas Abbey has lost his brave battle after suffering a colic this morning.
"Surgery revealed a severe strangulating colon torsion that was unviable and he had to be euthanized on humane grounds."
The son of Montjeu faced a multitude of problems as he strived to recover from a fractured pastern he sustained last summer.
O'Brien's star had to be retired after suffering a leg injury on the gallops at Ballydoyle when being prepared for the King George VI And Queen Elizabeth Stakes.
Just two days after he underwent a major leg operation at Fethard Equine Hospital last July, St Nicholas Abbey had to have emergency surgery to be treated for colic.
He then had a further setback in August when it was discovered a steel weight-bearing pin in a cannon bone had broken, and then had a further serious blow in October with "mild laminitic changes in the left fore".
St Nicholas Abbey will be buried at Coolmore, who added in the statement: "This is extremely unfortunate as St Nicholas Abbey had been in terrific form.
"The laminitis was resolving very well and the fracture had healed better than expected.
"Coolmore would like to thank the surgeons, the international experts and all the staff at Fethard Equine Hospital who gave him such excellent care 24/7. We would also like to thank the multitude of well-wishers."