Frankie Dettori will be out for around three months after undergoing surgery on the ankle he broke on Wednesday.
The 42-year-old, who had been set to ride leading Prix de l'Arc de Triomphe contender Treve on Sunday, was unseated from his horse Eland Ally while heading to the start of a race at Nottingham.
Initial on-course prognosis suggested he had suffered only soft-tissue damage but X-rays subsequently showed a break.
"It is looking like three months out," said Dettori's spokesman Peter Burrell.
"As well as fracturing his ankle, he has also broken his talus bone and he has had a plate and five screws inserted.
"The doctors think he'll need a month in plaster, a month in a boot and then it will be a month or six weeks for it to fully heal after that."
Dettori hopes to be back to ride at the Dubai Carnival at Meydan in early January.
The three-time champion jockey, who returned from a six-month ban at the end of May after testing positive for cocaine, will miss the Arc for the first time in 26 years.
Dettori has won the big French race three times previously, for his former Godolphin employers and trainer Saeed bin Suroor - with Lammtarra (1995), Sakhee (2001) and Marienbard (2002).
Unbeaten Treve, rated about 4-1 second favourite behind Japanese market leader Orfevre (2-1), will now be ridden by Thierry Jarnet at Longchamp.
Jarnet rode the filly to victory on her first three races before she was sold to Dettori's new boss, Sheikh Joaan Al Thani, and has won the Arc twice (Subotica 1992, Carnegie 1994).
Trainer Criquette Head-Maarek said: "He [Jarnet] knows the filly very well and won the Prix de Diane (French Oaks at Chantilly) on her in June.
"It is bad luck for Frankie, but that is life in racing."
In September, Dettori secured his first Group One winner since returning from his drugs ban when riding Treve to victory in the Qatar Prix Vermeille, also at Longchamp.
Dettori had been banned from racing for six months last December after testing positive for cocaine at Longchamp last year.
The British Flat season finishes on 9 November.