The Wallabies have suffered another injury scare, with hooker Tatafu Polota-Nau taking a tumble at Thursday's captain's run in Wellington.

Polota-Nau finished the session with ice on his knee but the Wallabies are confident he'll be fine to tackle the United States in their Rugby World Cup pool match on Friday night.

With Saia Faingaa out with the flu, reserve Stephen Moore is the only back-up hooker available to the Wallabies should Polota-Nau'sknee blow up overnight.

With five of their 30-man squad not considered for selection this week, and skipper James Horwill resting his battered shoulder, the Wallabies have been at pains to play down their injury troubles.

Prop Ben Alexander said the ice pack applied to Polota-Nau's knee was merely precautionary.

But it is hard to truly know the extent of the damage, if in fact there is any beyond wear and tear, because training sessions are held behind closed doors except for the first 15-minute warm-up period.

The Wallabies have also been confident this tournament that flanker David Pocock (back) and winger James O'Connor (hamstring) would be able to play in matches despite missing training sessions - and neither was able to back up.

If Polota-Nau succumbs to his apparent injury either before or during Friday night's match, Moore - who missed the weekend loss to Ireland with a gastro illness - would take over and one of the other front-rowers would presumably be on standby as a stop-gap hooker.

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