FREMANTLE defender Michael Johnson is expected to miss four to six weeks after undergoing ankle surgery on Saturday, but captain Matthew Pavlich could make an early return from a calf injury against Adelaide.

Pavlich injured the calf against Collingwood in round 13 and was expected to miss two to three games. However, he will now be tested ahead of this week's trip to face the Crows after missing just one game.

Johnson injured his right ankle in the side's round 10 loss to Richmond but played on and was only expected to miss two weeks.

Fremantle football manager Chris Bond confirmed Johnson would miss up to six weeks, while tough defender Brock O'Brien, who has been battling a quad injury, will miss the rest of the season.

The 21-year-old has suffered hamstring, shoulder and foot injuries since coming to the club with pick No.52 at the 2006 NAB AFL Draft and has managed just three games.

Fremantle has had a steady stream of injuries to key players this year and defender Antoni Grover said it had definitely had an impact on the club’s season.

"We're missing a lot of seasoned players," Grover said from Fremantle Oval on Tuesday. "It has given some guys opportunities [though]."

Fremantle has debuted nine players this season, but coach Mark Harvey hinted after Sunday's 15-point loss to Carlton that some of them may need a rest.

As Carlton hauled in a 36-point second quarter lead, the first-year Fremantle players ran out of legs, but Grover said his young teammates had been a huge success this season.

"If you ask me how they've been this year, out of 10 I'd give them 10," he said.

"They've grown as leaders within the club and been able have an impact in games.

"(They) have probably only done half the pre-season in their first year – especially the rookies that got drafted in December.

"There's no doubt over the next couple of years they'll be able to run out the seasons a lot better."

Fremantle faded badly against Collingwood in round 13 and gave up a fourth-quarter lead against Geelong a week prior, but Grover denied the club's second half lapses were a mental issue. 

"[It was] a very disappointing loss and the guys really felt it after the game – it's one that definitely slipped away from us," he said.

"Our focus going out after three quarter time was to win the game – there were no negative thoughts.

"You don't think about losing, we're thinking about winning, we're still trying to make the play. I don't think it's mental."