Rookie Berrick Barnes will be thrust into a World Cup baptism of fire against Wales on Saturday (at 1100 AEST) if veteran playmaker Stephen Larkham fails a desperate morning fitness test.

Wallabies coach John Connolly has rated Larkham just a 50-50 chance to play after he aggravated a knee ligament injury during final training in Cardiff on Friday.

The seemingly-indispensable 33-year-old was sent to hospital for scans but the relieving results failed to show any new strain.

Connolly will decide on Saturday morning, just five hours before the match at Millennium Stadium, whether to play 102-Test veteran Larkham in the pivotal pool B clash.

Larkham will have a private fitness test in Cardiff at 8am local time (1700 AEST)

The winner is almost certain to top the pool and book an easier quarter-final clash, probably against the loser of Saturday night's England-South Africa blockbuster in Paris.

Although Barnes made his Test debut off the bench last week, he only played 22 minutes in the 91-3 cakewalk of Japan in Lyon.

But the enthusiastic 21-year-old's impressive two-try showing has lifted him past Matt Giteau in the pecking order as Larkham's No.10 back-up.

Connolly feared the worst when Larkham's knee locked up when kicking at the end of the 60-minute captain's run at the Welsh Institute of Sport.

"He's had scans and it showed it was just aggravation, nothing more serious than that," the coach said.

"He'll have a run tomorrow morning and we'll make a decision after that about whether he plays.

"It's probably 50-50 at this stage but it's settled down well.

"This close to the game you do fear the worst and it did lock up on him but it has freed up now so he'll get treatment for the rest of the day."

Asked whether his five-eighth had to be 100 per cent fit to play, Connolly said the selectors had yet to decide whether they would take a risk.

Even if Larkham is proved fit, Connolly indicated Barnes would come on the bench.

That could be at the expense of utility backs Scott Staniforth or Julian Huxley, or even vice-captain Phil Waugh who was left out against Japan.

Barnes was seen as the luckiest member of the 30-man squad picked for the tournament but has impressed coaches and teammates alike with his work ethic and training form.

Connolly said he was comfortable in thrusting the youngster into the play-making hot seat.

"We are (comfortable)," he said.

"Things won't change. One player will drop out and one will go in and things will go on as normal.

"We wouldn't play him if we didn't think so."

He also said the move would best keep backline combinations, rather than reshuffling with Giteau moving from inside centre to flyhalf.

"That was the reason," Connolly said. Giteau three or four weeks ago was the back-up 10 and still may be long term but he hasn't trained there all week.

"He's trained well at 12 and Berrick's alternated all week at 10 and done very well and we're happy with the way he went last week.

"It won't change our plans, one will slide out and one will slide in."

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