St Kilda coach Ross Lyon believes the Saints' Friday night clash with Fremantle could define their AFL season.

Despite being heavy favourites, after eight wins in their past 10 games, Lyon expects a "ferocious" clash.

"We've done a lot right over the last 10 weeks, but in the ensuing five weeks this is the most important game," Lyon said on Wednesday.

The Saints have won their past five games straight to sit seventh after a slow start to the season.

The Dockers are eighth, but have not won in Melbourne this season and have just 23 fit players to choose from.

But Lyon did not expect an easy night in what is a key game.

"We're on red alert here, we're not taking anything for granted," he said.

"It's that classic eight-point game."

St Kilda will be missing suspended fullback Zac Dawson, who has been in impressive form, with Lyon still to decide on his replacement.

"They're a little bit shorter than they've been so we might stay mobile," the coach said.

"But if we're looking at tall defenders, Tommy Simpkin who's played a couple for us, Tommy Walsh, who's really pushing for a debut and then there's Jason Blake, who's a 196-game defender for us.

"They're the three obvious ones."

He said veteran defender Steven Baker could be recalled to handle dangerous Dockers forward Hayden Ballantyne.

St Kilda skipper Nick Riewoldt missed the club's 20-point win over Gold Coast through suspension, although Lyon believed that might have proved a blessing.

"He's certainly refreshed and keen ... he trained on Monday and felt a million dollars," he said.

"They've got some pretty good defenders, (Luke) McPharlin and those types to play on him, and (Alex) Silvagni did a great job on Buddy (Lance Franklin) so he's going to have some challenges."

Lyon said there were several keys to the Saints' surge into finals contention.

"It's a combination of competing, aggressive ball movement and appetite for the contest.

"There's been more consistency of effort and it's all started on the track."

Lyon expects veteran ruckman Michael Gardiner, who's yet to play a senior match this season, to take a step up in the VFL this week with a view to having one last crack at senior selection.

"He's only going to get one go at it, so if we bring him back too early and he was to have a soft tissue (injury), that would be it for Michael."

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