FREMANTLE'S ruck stocks have been dealt a blow with Zac Clarke likely to be sidelined until the second half of the home-and-away season after another setback with his troublesome knee.

Clarke has been forced to have surgery for the third time since the end of last season to fix fractures in the patella of his left knee.

The 26-year-old big man played through significant pain last year and underwent a knee operation after playing in Peel Thunder's maiden WAFL premiership.

He then aggravated the injury in the gym last December, which required him to spend six weeks in a brace. 

Clarke hoped to make his comeback during the JLT Community Series but is now facing a lengthy stint in rehabilitation.

"The medical advice we were provided after Zac’s recent surgery is that his rehabilitation phase means he will be available to resume full training in the second half of the season," Fremantle general manager of football operations Chris Bond said. 

"We will continue to monitor and assess Zac’s progress regarding his likely return date." 

Clarke's fresh bout of surgery is a sizeable setback for the 101-game veteran, who signed a one-year contract with Fremantle at the end of last year coming off two modest campaigns.

Since playing 21 games in 2014, the Victorian has been in and out of Ross Lyon's side and was the subject of trade speculation last year.

In better news for the Dockers, veteran ruckman Aaron Sandilands is on track to return from a calf injury during the JLT Community Series.

Sandilands is expected back in full training next week, while fellow big man Jonathon Griffin has been a strong performer on the track after overcoming Achilles soreness. 

Hulking draftee Sean Darcy has also enjoyed a strong summer and has been named in the Dockers' 29-man squad for Saturday's pre-season clash against West Coast in Geraldton.