Young forward Jesse White and former Canadian Test rugby representative Mike Pyke are vying to fill the long term gap left by injured ruckman Mark Seaby as AFL leaders Sydney prepare to tackle the "hardest gig in footy."

Sydney coach Paul Roos revealed on Monday Seaby would be out for up to 12 weeks and would undergo an operation on Tuesday on the broken left ankle he suffered early against Brisbane on Saturday.

Shane Mumford will lead Sydney's ruck division when he plays his first game against his former club at Skilled Stadium next Sunday.

Sydney have lost their last six games against Geelong and haven't won at Skilled Stadium in their last six visits since a 41 point victory in 1999.

Former West Coast premiership player Seaby has formed a highly effective ruck tandem with fellow recruit Mumford in the first six rounds of the season.

Roos said Seaby had been superb through that period in which Sydney have won five games.

"He has been terrific, so it is a bit of a blow," Roos told reporters.

"I thought when Jesse (White) went in the ruck (against Brisbane) he did really well and Pykey has been in really really good form in the seconds."

Roos said he was wary of sacrificing White's development as a forward and stressed Pyke had improved significantly from his rookie season in which he made six senior appearances.

"He is certainly a lot further advanced than he was last year, his ruck work has been terrific and he's finding a lot of footy around the ground as well," Roos said.

While Mumford has shared starting duties with Seaby at Sydney, he will clearly be Sydney's senior ruckman in the weeks ahead.

Roos believes 23-year-old Mumford, who has played just 27 senior games, is ready for the responsibility of being Sydney's senior ruckman.

"I think if that had happened in round one, he probably wouldn't have been ready, because he still hasn't played a lot of footy," Roos said.

"He has really built into this season nicely. I thought on the weekend he was again probably in our top half dozen players and he really worked hard.

"He went forward and took a mark, so he's certainly developing as a player as we had hoped."

Roos was under no illusions about the task facing his side on Sunday, following the Cats' 108 point demolition of Richmond.

"Its the hardest gig in footy, there's no doubt about that," Roos said.

"Their best is still better than anyone else in the competition. We are going to have to be absolutely spot on to give ourselves any chance to win down there."

Roos believed Geelong enjoyed one of the few remaining home ground advantages in the competition and any side that aspired to be beat them needed to win more than 50 per cent of possession.

"They are the hardest team to get the ball back off in the lead because their skills are just so good and they are so smart," Roos said.

"Their players know where to go, they are well coached, they've got a good game plan, so if you don't get enough of the ball, it's lights out."

Seaby apart, Roos said Sydney's only other injury doubt was midfielder Dan Hannebery, who was 50-50 to play because of a shoulder problem.

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