West Coast's horror season took another turn for the worse after key midfielder Chris Masten injured his knee in Sunday's 38-point AFL loss to bitter rivals Fremantle at Subiaco Oval.

And veteran Andrew Embley is almost certain to join Masten on the sidelines after being reported for a dangerous head-high bump on Fremantle on-baller Rhys Palmer.

Masten will undergo scans on Monday to determine the extent of the damage but Eagles coach John Worsfold confirmed the nippy 21-year-old had re-injured his posterior cruciate ligament.

The loss of Masten is a massive blow for the Eagles, who are already without Daniel Kerr (hamstring) for most of the season.

"He (Masten) had the same thing last year which was a long time out," Worsfold said.

"But sometimes they're only very minor and it's only two or three weeks out.

"That's the range at the moment, until it's scanned they (the medical staff) haven't said anything."

Embley charged into a bent-over Palmer just six minutes into the match, an incident which incited a mini melee and left the 21-year-old groggy and with a sore neck.

Masten's injury - on his 21st birthday - and the report of Embley capped off a disastrous day for the Eagles, who shot out to a 23-point lead eight minutes into the second quarter but failed to kick another goal until 21 minutes into the last quarter.

Fremantle kicked 11 unanswered goals during that time to run out convincing 17.9 (111) to 10.13 (73) winners, their sixth derby triumph on the trot.

The Dockers' fifth win of the season sees them join Sydney, St Kilda and Collingwood on top of the ladder, while the Eagles are now 1-5 ahead of Saturday's clash with Hawthorn.

Fremantle coach Mark Harvey played down his team's hot form ahead of tough games against Brisbane (Gabba), Collingwood (home) and Sydney (SCG).

"You learn not to get carried away with where you're at," Harvey said.

"We are maybe an injury or two away with one of our star players and then all of a sudden you have to reassess everything."

Worsfold conceded his team's finals aspirations were all but over.

"We always said we would like to push up and try to fight for it but reality for us now is, with a couple of injuries, let's just try and make each week a real contest and see how many wins we can come away with," he said.

Fremantle will monitor the fitness of Stephen Hill, Hayden Ballantyne, Alex Silvagni and Chris Mayne during the week after all suffered minor leg injuries during the first half before playing out the match.

Michael Barlow tallied 25 possessions, seven tackles and one goal to win the Ross Glendinning medal as best afield.

Matthew Pavlich was kept relatively quiet but still kicked five goals, while Paul Duffield's run from defence proved crucial in the win.

Quinten Lynch was kept goalless by Luke McPharlin while Josh Kennedy (three goals) only popped up late after a dirty day and Mark LeCras (two goals) was well held by Chris Tarrant.

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