Chris Tarrant faces a race against time to be fit for Fremantle's AFL finals campaign after suffering a suspected medial ligament injury in Sunday's 75-point western derby win over West Coast.

Tarrant, who had missed the previous five weeks with a heel complaint, was left writhing in pain late in the first quarter after his knee twisted awkwardly in a clash of bodies low to the ground with Eagle Andrew Embley.

The 29-year-old will undergo scans on Monday to determine the extent of the damage.

But with just four rounds remaining before finals and the minimum recovery period for a medial ligament injury about a month, Tarrant may struggle to break back into the side in time for their September campaign.

"We'd like to think it's three or four weeks (out) but we'll wait for the full diagnosis in the morning," a hopeful Harvey said after the crushing 24.16 (160) to 13.7 (85) victory, Fremantle's seventh derby success on the trot.

"It depends on the grading of it.

"He's upbeat.

"He naturally just wants to be part of the team and the way they're winning at the moment.

"He's a player that comes up quite quickly so he doesn't need a lot of game time (before returning to the senior side).

"Even today you saw he didn't look like he had been out (for five weeks prior)."

The injury to Tarrant was the only low point for Fremantle in yet another memorable derby triumph.

The Dockers were never troubled after booting seven goals to nil in an inspired first-term onslaught, with Ross Glendinning medallist Aaron Sandilands (33 hit-outs, 22 possessions), Hayden Ballantyne (six goals) and Matthew Pavlich (four goals) pivotal players throughout the afternoon.

Fremantle's 12th win of the season lifts them four wins and sizeable percentage above ninth-placed North Melbourne, all but guaranteeing the Dockers a return to finals action for the first time since 2006.

And with Carlton, Sydney and Hawthorn slumping to defeats over the weekend, the Dockers are now almost certain to finish in the top six - guaranteeing them a home final - and are still only percentage adrift of fourth spot.

Fremantle round out their season with winnable games against North Melbourne (away), Sydney (home), Hawthorn (away) and Carlton (home) but Harvey refused to talk up his team's chances of sneaking into the top-four.

"We've got four weeks to go and if you go and have a look at the sequence of who we play and then whoever's in the top four at the moment and see who they play, you'll probably work out where everyone finishes," Harvey said.

"All we're doing is trying to put ourselves in a position.

"The more we win from here on end the better position it's going to be.

"Up until this stage we've had an exciting season."

Eagles sharpshooter Mark LeCras kicked five goals after Tarrant went off injured but West Coast's inexperienced line-up simply couldn't cope with Fremantle's relentless pressure.

"It was a very slow start, just outgunned really from the start with the intensity and couldn't claw our way back into it," West Coast coach John Worsfold said.

Eagles defender Shannon Hurn was a late withdrawal due to a slight hamstring injury while Chris Masten rolled his ankle during the match and is likely to miss Saturday's wooden spoon showdown against fellow strugglers Brisbane in Perth.

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