Australia's Bledisloe Cup preparation suffered a serious stumble last night when Wallabies captain Stirling Mortlock was forced out of Saturday night's Sydney Test because of a head knock.

Although Mortlock was involved in yesterday's training session at Coogee Oval, the Wallabies team management met last night and decided their Test skipper had not recovered sufficiently after being concussed during the second half of their 16-9 victory over South Africa in Perth last weekend.

Former Sydney Roosters league player Ryan Cross is scheduled to make his first starting-XV Test appearance in Mortlock's place at outside-centre, while George Smith will take over as captain.

"Stirling was concussed last weekend, and we had hoped that, with time, the symptoms would resolve themselves," Wallabies coach Robbie Deans said. "But that hasn't happened soon enough for this match, so we've made the call early. George [Smith] took over once Stirling left the field in Perth and got the team over the finish line, so we've opted to maintain continuity by leaving him in charge."

While the loss of Mortlock is a considerable blow, considering that in many recent Test encounters against the All Blacks he has succeeded in showing up opponents Luke McAlister, Carlos Spencer and Leon MacDonald, it did not prompt an outcry of despair from the Wallabies camp last night.

They believe that Cross is the best of replacements, showing this season off the Wallabies bench and with the Western Force that he is a brilliant finisher, a pugnacious defender and intimate with all the right running lines. The Wallabies also look upon his direct All Blacks opponent, Richard Kahui, as a possible weak link. They had been expecting the more experienced Conrad Smith to start in the All Blacks midfield.

The Wallabies are preparing to see Richie McCaw lead out the All Blacks, while they have also not lost hope that their aggressive second-rower James Horwill will be among the forward contingent aiming to keep the world's best openside breakaway in his place.

Although McCaw, returning from a high ankle sprain, was yesterday bracketed in the All Blacks pack with Daniel Braid, the Wallabies are already preparing their game plan and tactics around the belief that McCaw will play the first half and beyond.

As Wallabies breakaway Rocky Elsom said yesterday: "He [McCaw] will definitely strengthen their side, and it will be a massive boost for them."

Wallabies inside-centre Berrick Barnes is anticipating that McCaw will be running at him from the first minute.

"He is their captain, so I wouldn't be surprised to see him out there," Barnes said. "He's been rated as the best No.7 in the world, but mind you our flankers are going pretty well at the moment. He's obviously a class act, and they'll be very happy to have him."

That McCaw needed containing was one of the few certainties that emerged from a Wallabies training session yesterday. It prompted many doubts over the exact make-up of the Australian side, which will be announced this afternoon. The Wallabies had to field a makeshift pack in its scrummaging and lineout work because three key forwards - No.8 Wycliff Palu, Horwill, and Elsom - did not train.

It was also a worrying sign that at the end of the session Horwill, who had an ice pack on his left foot, was seen slowly hobbling across Coogee Oval on his way back to the team hotel.

Horwill certainly did not look to be in any condition to play in a Test match if it was being staged in the next 48 hours, but Wallabies officials were last night positive he would be able to play on Saturday night.

The obvious alternative for Horwill if his foot does not improve is Australia's premier second-rower, Dan Vickerman, who is steadily finding his fitness following ankle surgery. Vickerman trained for the full session yesterday.