Cats recruit Sam Menegola also has a knee injury, but they are confident he will return early in the season.
Thurlow, 22 this month, tore the anterior cruciate ligament in his right knee in Saturday’s pre-season win over Essendon.
They have ruled him out for this season.
Thurlow was stretchered off the ground at Shepparton after he collided with Essendon opponent and former Cats teammate Mitch Brown in the third term.
He said on Twitter the injury was shattering and hoped next season would come quickly.
— Jackson Thurlow (@JThurlow40) March 7, 2016
The Cats youngster enjoyed a breakout season last year, forcing his way into Geelong’s best side with 19 matches.
Thurlow also earned a Rising Star nomination.
“His development came along in leaps and bounds last year,” said Cats football manager Steve Hocking.
“He was definitely in the mix to become a permanent member of our back six this season after showing great promise throughout the pre-season.”
Geelong’s summer had been flawless before the bad news about Thurlow.
They are bullish about their prospects, despite dropping out of the top eight last year for the first time since 2006.
Star onballer Patrick Dangerfield leads an impressive list of recruits that also includes ruckman Zac Smith, utility Lachie Henderson and veteran midfielder Scott Selwood.
They have also welcomed back highly-rated players Daniel Menzel and Nathan Vardy from long-term injuries.
Thurlow also had to overcome a serious kidney injury before last year’s strong form.
The first-round pick in the 2012 draft has played 29 senior games since his 2013 debut.
Also on Monday, Geelong confirmed that Menegola suffered a posterior ligament tear to his left knee in last month’s pre-season game against Collingwood.
Hocking said it was a minor setback.
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