Collingwood coach Mick Malthouse has conceded he's taking a punt by going in with just one specialist ruckman, Josh Fraser, against Fremantle behemoth Aaron Sandilands at the MCG on Saturday.

Fraser's backup, Cameron Wood, was one of three players dumped this week from the Magpies line-up that comfortably accounted for the Swans in Sydney last Saturday night.

The Pies' selection policy is justified, Malthouse believes, by the fact that Sandilands is virtually guaranteed to get first hands to the ball at the stoppages no matter who he's up against.

"He's almost immovable, he's got 211 centimetres of height, he's got a reach like a sick dog, so how do you get near him," Malthouse pondered.

"It's a battle for two ruckmen and we've just elected to go with Josh and we'll see if we can cater for the other with (Chris) Dawes, (Travis) Cloke and (Leigh) Brown."

"Is it punt material? I don't know whether it's a total punt."

"We think we can get better value out of Josh probably playing more time in the ruck then he does sitting on and off the bench."

Malthouse stressed that Wood, whom he described as slow to mature, would play a key role for the club in time.

"You have to have patience ... and we haven't forgotten that Cameron Wood is only a young man who needs time and when he gets to the power stage, when he puts some kilos on, he will be a very good ruckman for us," said Malthouse.

"He's a smart player, a one-touch player, can kick goals, knows where to position himself - it's just a matter of time."

Quizzed about why the club would make four changes overall - three of them unforced - to a line-up that had won four games straight, Malthouse said: "We've got to keep pressure on players and pick the best side that's appropriate for the opposition."

"We always look at our side and think that if the side has won or lost it's still looked at in the same position as trying to pick the best side for the next week."

"And under those circumstances a couple of those players have made way because of the form of the other players who are coming in."

Malthouse said Paul Medhurst, named as an emergency for Saturday, would return to the senior line-up only when he can prove that his form is superior to that of the player he replaces.

"There's no automatic selection for anyone ... and there's no walk-up start for him," said Malthouse.