Berrick Barnes took over the playmaker's reins with aplomb after coming on as a replacement in Australia's bruising World Cup Pool C victory over the United States.

In only his eighth game back from a series of head injuries that left him suffering with debilitating migraines, Barnes slotted in at flyhalf, starter Quade Cooper dropping to fullback with Kurtley Beale off the park.

And what a difference Barnes made, giving the backline an immediate steadying hand, unlike Cooper, whose obvious skills were again mired by theatrical, forced moves that did not come off.

"That was one real positive to come out of the game," Australia coach Robbie Deans said of Barnes' performance.

"Berrick hadn't played at that level for a long time and at the back of his mind there would have been some demons but he did well."

Barnes has sat out a lot of rugby recently, missing most of the Super Rugby season.

He suffered concussion in the Waratahs win over the Queensland Reds in February and has been troubled by what he dubbed "footballer's migraine", a condition that includes a throbbing head, sensitivity to light, nausea and ringing in the ears.

"It's exciting to be part of the World Cup and you obviously want to play and last night I did," said the 25-year-old Brisbane-born Barnes.

"It was definitely good to get out there."

Barnes, who spent his first senior season playing professional Rugby League for Brisbane Broncos, admitted he had doubts about his ability to return to Rugby Union after his health problems.

"Concussion is different for different guys. I had more of a footballer's migraine and it tended to drag on more," he said.

"I had to take a bit of time out, and I'm thankful I did that. I'm feeling good now and this is my eighth game since I've come back so I'm pretty positive that if I keep doing what I'm doing there shouldn't be any recurrences but time will be the judge there."

Barnes added: "I never had any problems until this year.

"It got to the stage where I wasn't of the ability to keep playing, I lost all confidence, and to be honest I had issues off the field, even getting around.

"But now I'm fine and comfortable that what I'm doing is working.

"I had some serious doubts [about a comeback]. I had a break and I came back when I was feeling right.

"At the end of the Super 14 the last thing on my mind was the World Cup and I'm pretty rapped at being here."

Wallabies coach Deans was thrown a curveball when three centres suffered serious injuries in Australia's 67-5 win over the Americans on Friday.

Rob Horne sustained a fractured cheekbone, Pat McCabe dislocated his shoulder and Anthony Fainga'a was knocked out, with further tests to be carried out on the trio ahead of next Saturday's final Pool C match against Russia.

Deans will at least be happy to have Barnes ready and raring to go, the demons of seven months of migraines hopefully cast aside for good.

AFP