Champion chaser Sire De Grugy has been ruled out until next year, with a return most likely in February.
The horse, ridden by Jamie Moore for trainer father Gary, won six out of seven races last season.
But a hip injury means he misses facing another Queen Mother Champion Chase winner, Sprinter Sacre, in next month's Tingle Creek Chase at Sandown.
"He definitely won't be running in the Tingle Creek and he won't run again this year," said Sussex trainer Moore.
Sire De Grugy, originally bought as a 50th birthday present for his owner Steve Preston, was voted jump racing's horse of the year in April.
His supporters, wearing the red and blue colours of Preston's favourite football team Crystal Palace, have added a splash of colour to the sport.