ROSS Lyon feels "guilty" for not giving Cam McCarthy enough aerial support in Fremantle's attack, after the mobile tall was whacked for his performance against Sydney last round.

Former Western Bulldogs and Richmond coach Terry Wallace blasted McCarthy's effort as "awful" after he went goalless from 11 touches and took two marks at the SCG.

McCarthy's form has dipped since spearhead Matt Taberner went down with a broken foot in round five, with the former Giant averaging eight disposals, 2.5 marks and a goal over the past four matches.

While emerging swingman Brennan Cox has offered McCarthy some respite, Lyon put his hand up for not giving the 192cm goalkicker enough help in the air.

"I feel a bit guilty with Cam because (of) the team we've picked," Lyon said ahead of Sunday's expected wet weather clash with North Melbourne.

"I take responsibility for Cam not having a lot of support, so that just flags (David) Mundy will play a lot more forward.

"We think with the support, when he's more freer and he's not feeling the pressure of being that spearhead and tall all the time, he then plays very good footy.

"I think as a match committee we've missed that a little bit and it's something we're conscious of going forward." 

The Dockers, who only scored 52 points against the Swans, also have Shane Kersten in the wings.

But Lyon has been reluctant to call on the former Geelong forward following the decision to reignite his career as a defender.

Kersten is on standby for close mate Joel Hamling, although the Dockers are confident the full-back will be fit to mind one of North's dangerous tall forwards.

Hamling passed a cognitive state test this week after suffering concussion against the Swans.

The Roos present the biggest threat to Freo from an interstate side this year at Optus Stadium, with Brad Scott's men boasting the League's stingiest defence, the Coleman Medal leader Ben Brown and a match-up for superstar Nat Fyfe in Ben Jacobs.

Lyon will encourage Fyfe to drift forward to take Jacobs, who recently held Brownlow medallist Dustin Martin to 16 touches, out of his comfort zone.

"Jacobs is doing a good job, isn't he? He's got some good scalps," Lyon said.

"He's got a big tank, beautiful kick. There's a lot to Jacobs. Nathan's going to have a fight on his hands and we'll need to help him.

"You try to take them out of their comfort zone.

"He's certainly an aerial threat, Nathan. He tends to mark really well in the wet – he marks as well in the wet as anyone I've seen.

"We would encourage Nathan to do that."

Fyfe's troublesome knee ligament has raised concerns for the Brownlow Medal favourite but Lyon was confident his captain could continue to manage the issue. 

He also revealed the Dockers won't blood speedster Stefan Giro on Sunday, believing the rookie needs more time at Peel after three games back from a knee issue. 

First-year midfielder Mitch Crowden has been rested and Freo will continue to monitor Andrew Brayshaw, Adam Cerra and Bailey Banfield.

"It's not an exact science. They're like lab rats, they get measured on everything and indicators for everything," Lyon said.

"But I think our form and their run will tell us most, and just their general demeanour, but they're really bubbly and enthusiastic."