The 34-year-old, who won the Liege-Bastogne-Liege classic race last year, sustained the injury when he fell off a mountain bike in Victoria's High Country on Sunday and will undergo surgery on Monday.
After the operation, Gerrans will rest for a "a week to ten days" before starting his preparations for the spring classics in Europe.
"Unfortunately it doesn't matter how well the recovery goes from here, I don't think I am going to be in any condition to be racing at a high level in January," Gerrans said in a news release.
"With that in mind I think it is better to take my time, make sure the recovery goes well and get all the rehab done so I don't have any ongoing issues.
"It's a big blow not to be able to line up in the national championships and Tour Down Under and defend my titles. They are races that I really enjoy and I know how important they are to the team."
Gerrans, who has won stages on all three major tours, raced to victory in the Tour Down Under in 2006, 2012 and 2014, also winning the Australian national championships in 2012 and 2014.
"Looking on the bright side, what it does do is force a bit of a break now and mean that I can work towards some goals later in the season and be a lot fresher," he added.
The Australian road racing championships take place in Ballarat from Jan 7-11, while the Tour Down Under starts in Adelaide on Jan 17.
(Reporting by Nick Mulvenney, editing by Patrick Johnston)
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