St Nicholas Abbey underwent emergency surgery for colic the morning of July 26, and his prognosis is "very guarded," owner Coolmore said.
The 6-year-old Montjeu horse had surgery July 24 at Fethard Equine Hospital for a fractured pastern. He appeared to be making progress before the latest development.
"Unfortunately St Nicholas Abbey had a major setback in his recovery during the night, suffering from bad colic," Coolmore said via Twitter.
St Nicholas Abbey, trained by Aidan O'Brien, fractured a pastern at the Ballydoyle training facility. Coolmore earlier stated the operation would be long and complicated.
"He will remain in intensive care for some time, and has many bridges to cross before he is considered out of danger," Coolmore said.
St Nicholas Abbey has nine wins, two seconds, and seven third-place finishes in 21 starts for earnings of $7,863,492. His victories include the Emirates Airline Breeders' Cup Turf, Investec Diamond Jubilee Coronation Cup (three times), Racing Post Trophy, and Dubai Sheema Classic Presented by Longines, all grade or group I events.