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.