STAR West Coast defender Jeremy McGovern has been ruled out of Saturday night's Western Derby, paving the way for a possible switch into defence for recalled veteran Jack Darling.

And Fremantle has confirmed skipper Alex Pearce will miss between four to six weeks with his latest arm fracture, while Michael Walters is out for three to four weeks with a hamstring injury.

McGovern, who had missed the past two games with a punctured lung and a fractured rib, was named to return against Fremantle when the teams were released on Thursday afternoon.

But the four-time All-Australian failed a fitness test on Friday morning, with speedster Jack Petruccelle instead thrust into the 23-man squad.

"He was close. Gov's pretty tough, as we all know," West Coast interim coach Jarrad Schofield told reporters on Friday. 

"We had to take it out of his hands a little bit in the end.

"In Tom Barrass' 150th game - he would have loved to have been out there with his partner in crime, but at the end of the day he just wasn't right."

Schofield threw up the option of playing Darling in defence in order to fill the void left by McGovern.

Darling was dropped two weeks ago for the first time in his 293-match AFL career when Schofield was appointed interim coach.

The 32-year-old will return alongside Elliot Yeo and Jake Waterman against the Dockers, but what role he plays remains up in the air.

"Someone will have to size up and play a bit taller," Schofield said of his defensive stocks. 

"Brady Hough can play taller. We've got some options.

"I keep mentioning about Jack Darling and his role - we'll see what happens there."

Schofield said it was a tough call to drop Darling a fortnight ago.

"To have that conversation was tough," Schofield said.

"We've just seen it with Brad Scott with Dyson Heppell (at Essendon).

"They're tough decisions, but Darling's gone about it the right way. 

"We want to find the balance between the exploration part around our youth and our list, and also we want to win games or football and we want to compete."

Pearce faces a race against time to play before finals after undergoing surgery on Thursday to fix a fracture in his left arm.

The 29-year-old also had surgery five weeks ago when he first broke his left forearm.

The latest fracture - which happened during his comeback game against Melbourne last week - occurred at the base of where the original plate was inserted.

Fremantle only has five home and away games left, meaning Pearce faces a battle to feature before finals.

"He had another plate put in, so he will get out of hospital (on Friday) and start his journey back," Fremantle coach Justin Longmuir said.

"We're thinking probably four to six (weeks)."

First-year defender Josh Draper has been recalled to replace Pearce.

Longmuir also gave an update about Walters, who suffered a hamstring injury during his four-goal performance against the Demons.

"It's the same hammy (that he injured earlier this season) but it's a different injury and he had some awareness during the game," Longmuir said. 

"So we got it scanned and got a worse result than we were expecting. 

"We're thinking three to four (weeks). So slightly worse than his other one, but we're hoping he gets back before the end of the season."