Flemenstar suffering from a lung infection
(Healy Racing Photos)
The Stephen Curran-owned eight-year-old, who finished second to Sir Des Champs in the Hennessy Gold Cup at Leopardstown on Saturday, was diagnosed with the problem on Monday.
He is instead likely to be aimed at the big Aintree meeting in April.
Casey told Press Association Sport: "He'll be out for two to three weeks.
"It's a bad lung infection. The vets found he had it today.
"He's going to be all right in the long term, but I would say Cheltenham is very unlikely now.
"The jockey (Andrew Lynch) thought something was wrong on Saturday and it looks like he was right.
"We'll probably look at Aintree now."
Flemenstar holds entries at Cheltenham in the Betfred Gold Cup, the Ryanair Chase and the sportingbet.com Queen Mother Champion Chase.
Anglo Printers, the Drogheda, Co. Louth based printing company has won a tender to supply racecards to all HRI owned racecourses.