The eight-year-old, who finished second to Sir Des Champs in the Hennessy Gold Cup at Leopardstown on Saturday, was diagnosed with the problem yesterday afternoon.
He is instead likely to be aimed at the big Aintree meeting in April.
Casey said: "He'll be out for two to three weeks. It's a bad lung infection.
"He's going to be all right in the long term, but I would say Cheltenham is very unlikely indeed.
"The jockey, Andrew Lynch, thought something was wrong on Saturday and it looks like he was right. We'll probably look at Aintree now."
Elsewhere, Grands Crus still has the World Hurdle and Ryanair Chase as possible Festival targets as he recovers from an ulcer problem.
Third in the King George VI Chase having had a breathing operation, the eight-year-old ran poorly again when pulled up in the Argento Chase at Cheltenham last month.
Trainer David Pipe said yesterday: "He was given an easy time of things after his race while we tried to get to the bottom of his lacklustre effort and he is now being treated for gastric ulcers.
"He is back on the gallops and looks well in himself so I will need to speak with his owners and decide the plan of action.
"Tomorrow there is a forfeit stage for the Gold Cup, Ryanair Chase and World Hurdle and I can confirm he will be scratched from the Gold Cup, but will be left in the Ryanair and World Hurdle."