WITH ZAC Dawson and Alex Silvagni sidelined through injury for the early rounds of the season, Fremantle defender Michael Johnson knew the tasks thrown his way were going to be big.

They don't come much bigger than Geelong powerhouse Tom Hawkins in his kingdom at Simonds Stadium but rather than shy away from the assignment, Johnson relished the task of playing a vital role in the Dockers' 44-point win smashing of the Cats on Sunday.

"I've known for a couple of weeks that I've had to step up and play that key defender role, which I'm happy to do," Johnson told AFL.com.au.

"We've got some great backman down there - if we do get into trouble, any of them can step up and play their role.

"It'll be good to get the boys back (Dawson and Silvagni) so I can (get) back up the field but I'm just playing my role for the team and that happens to be as a key defender which is OK with me."

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Despite giving away a few centimetres in height and about 15kg in weight, Johnson battled manfully against the Cats spearhead and limited him to two goals, 11 disposals and more impressively just five marks.

As usual, Johnson's rebound from defence – 24 disposals, nine marks and six rebound 50s – was also important.

Of course the midfield pressure and the fact Ross Lyon decided to deploy Garrick Ibbotson in a 'plus-one role' behind the ball helped Johnson's cause.

But even with the help of his teammates, Johnson was still able to win his fair share of one-on-one tussles with the big Cat.

Lyon said the pressure of Fremantle's midfield in recent weeks had allowed the Dockers to chop and change their defensive structure based on match-ups.

"We thought Geelong would play three key forwards. We're happy being a little undersized at the moment because it gives us real run and bounce out of there," Lyon said.

"As long as the pressure up the field is strong, we'll defend it quite well."

After a flight out of Melbourne on Sunday night, Johnson said the Dockers would quickly turn their attention to West Coast for Sunday's derby.

"It's a massive occasion for West Australians. You walk down the street and there's always a lot of purple and a lot of blue and yellow," Johnson said.

"There's a lot of families, when it comes to derbies, that their households are pretty much split down the middle and it's just a great occasion.

"If we play the way we have played the last two weeks, we should be OK. But we know that they're coming off a really good win against Carlton and they're still a quality side."