Brennan Cox is assisted off the field during the round one match between Fremantle and Brisbane at Optus Stadium, March 17, 2024. Picture: AFL Photos

FREMANTLE is bracing to be without key defender Brennan Cox for the first half of the season after confirmation he would require surgery for a serious hamstring injury.

Cox grabbed at his right hamstring after a marking attempt in the final quarter of Sunday's win against Brisbane, with the 25-year-old undergoing scans and visiting a surgeon on Monday afternoon.

Coach Justin Longmuir confirmed the important backman would require surgery for the injury, likely sidelining him at least three months.

"He's going to see a surgeon this afternoon, but it looks like he will require surgery on that hamstring to be able to repair it," Longmuir told Fox Footy's AFL 360 on Monday.

"[He'll be sidelined for] somewhere between 12 to 15 weeks, I would imagine."

Longmuir said the Dockers required more information on fellow defender Oscar McDonald's knee injury after the former Carlton and Melbourne backman hyper-extended his left leg while landing awkwardly in the third quarter.

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"He's gone to see a surgeon this afternoon as well. He's got a little bit going on in that knee," the coach said.

"I'm not going to say what it could be, because it could be a number of things.

"It was his first game at the club, and he was playing a really important role for us and has had a super pre-season.

"Hopefully we get a better result there."

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Longmuir was confident the Dockers had depth to call on, with 197cm backman Josh Draper an option and small defender Brandon Walker close to being available after building match fitness in a WAFL practice match at the weekend.

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The Dockers could also shift Luke Ryan and Ethan Hughes into roles on taller players if needed to support captain Alex Pearce.

Cox had played forward against Brisbane, with former Bomber Patrick Voss and experienced key forward Matt Taberner the Dockers two main options to call on in attack if looking for a tall replacement.