Casey Stoner says his chances of retaining his MotoGP title are "finished" after breaking his ankle at Indianapolis last weekend.
Stoner, who will fly back to Australia for surgery and will miss Sunday's Czech Grand Prix, says he has "no idea" when he will return.
"Until we've had the surgery we won't know," said the 26-year-old Australian, who retires at the end of the season.
"It's disappointing news but it's finished our championship."
Stoner currently lies in third place in the standings, 39 points behind leader Jorge Lorenzo with seven races remaining.
Stoner took fourth place in Indianapolis despite suffering torn ligaments, bone damage and fractures to his ankle in a crash in qualifying. But he has been advised by doctors to have the surgery to prevent permanent damage.
The Honda rider, who won the championship in 2007 and 2011, said: "The plan is to get back to Australia as we have to wait for the inflammation to go down before having surgery. We already have everything lined up to see what we can do and then try to get back.
"Normally my doctor doesn't have a problem with me riding with injury but as soon as he read the report he advised me against riding in Indianapolis. All the reports have been the same, that I need to get off this foot as soon as I can."
Stoner hopes he can return a race or two before his home GP at Phillip Island on 28 October.
"To have these kind of disappointments is frustrating, especially in my last year, but that's racing," he said.
"I'm not in this championship to finish on a fairytale. I'm not always going to keep waiting for a fairytale ending."
Title rival Jorge Lorenzo, who is favourite to regain the crown he won in 2010, said: "I wish Stoner a quick recovery.
"It won't be the same in the next races, and for sure it will be easier for Dani Pedrosa and I to win."
British rider Cal Crutchlow said on Twitter: "Sad to hear Casey Stoner cannot ride but after seeing his ankle yesterday I can't believe he rode in Indy!
"Hope he's back soon with the other guys too."
Nicky Hayden and Hector Barbera have also been ruled out of this weekend's Czech Grand Prix