Sydney has suffered a serious blow ahead of Saturday night's crunch AFL clash against the Lions in Brisbane, with defender Tadhg Kennelly ruled out with a knee injury.

Kennelly, who has already missed six games throughout the season due to knee trouble, picked up another knee injury at training on Thursday night.

The extent of the problem is not currently known, with coach Paul Roos saying the Irishman had scans last night and will be assessed next week.

Roos is preferring to focus on the game against the Lions which sixth-placed Sydney needs to win to maintain their late push for a top four berth.

"At this stage we really don't know how long the injury will keep Tadhg out," Roos said.

"Our major focus at this stage is on tomorrow night's game against the Lions.

"We will look further into the situation with Tadhg's knee early next week."

Rookie Ed Barlow will now make his debut, having being elevated to the senior list this week after Stephen Doyle and Daniel Currie were placed on the long-term injury list.

The match is vital for both teams, with Sydney trying to improve on its 11-8 record and Brisbane aiming to continue their drive towards September and rebound from last week's defeat to Hawthorn.

The Lions, who sit just inside the eight on percentage with nine wins, nine losses and a draw, haven't beaten Sydney in their last six attempts.

AAP