Flemenstar has been ruled out of Sunday's John Durkan Memorial Chase at Punchestown.
The Tony Martin-trained eight-year-old won the Grade One event last season, but will not be able to defend his crown after meeting with a setback.
Owner Stephen Curran said: "He's got a bit of a knock, I'd say he knocked into himself.
"We don't know how bad it is, we thought he would be OK but he's too good a horse to be taking a chance with.
"There's a bit of swelling there, it might not be too serious but we don't really know at this stage. We'll see how he is and we'll know more in five or six days. We'll mind him and be patient, and he'll get the time he needs.
"He was in the form of his life, in tremendous form. He did a piece of work the other week that was unbelievable.
"It's just bad luck, we were really looking forward to it."
Flemenstar's defection leaves a field of just three for the Punchestown feature, with Willie Mullins represented by Sir Des Champs and Arvika Ligeonniere and Robbie Hennessy running Rubi Light.
The www.punchestown.com Rated Novice Hurdle gets things under way at 12.30pm and Ange Balafre bids for a four-timer following a victory at Gowran last month.
Given Willie Mullins' excellent 45% strike-rate in bumpers this season, Black Hercules warrants the utmost respect in the closing Old House Kill (Pro/Am) Flat Race, although Sucker Punch changed hands for & pound;105,000 following his point-to-point success and it should be an interesting battle.
Apache Jack attempts to build on a promising hurdling debut in the 2014 Annual Membership Maiden Hurdle, while Ruby Walsh is an interesting jockey booking for Robert Tyner's Daring Article in the Weatherbys Ireland GSB Handicap Chase on the seven-year-old's first start since scoring when last seen in May.