Fremantle defender Brennan Cox will be sidelined for a minimum of four weeks due to a stress reaction in his fibula.

The 25-year-old defender experienced lower leg soreness from last Friday's training session, which prompted the Club to seek further medical evaluation.

Cox was later sent for scans on Monday, which confirmed a bone stress reaction in his fibula.

As a result, Cox will now continue his pre-season training for the next month in an off-legs rehabilitation capacity.

Following completion of his off-legs program, Cox will then build his loading before being re-introduced to football with a plan to re-join the later stages of pre-season and be available for selection in round one.

Executive General Manager of Football Joe Brierty is confident that Cox will be able to maintain his fitness during his rehabilitation program.

"While it’s an unfortunate interruption to Brennan’s pre-season on the track, he will be able to continue cross-training during this off-legs window to maintain his conditioning before returning to football.” Brierty said.

“Brennan has the full support of our medical team to ensure that this setback can be managed appropriately and have him at full strength for the opening round.”

Fremantle will play the reigning grand finalists Brisbane for the round one clash on Sunday 17 March at Optus Stadium.