Lucky the Wallabies took a cast of thousands to the United States because without extensive and experienced backup they may have encountered the ultimate embarrassment in Chicago.

After an abominable first half, when numerous Test candidates appeared desperate to do everything they could to convince their coach, Michael Cheika, they weren't up to World Cup standard, the reserves bench saved the day after the break by bringing logic and decorum back to a pretty messy Australian game plan.

The lopsided score is a smokescreen as the Wallabies were diabolical for so much of the match. Their opening 40 minutes would rank among the most bewildering first halves of football this erratic national team has played in recent times.

Sean McMahon scored his first Test try for the Wallabies © Getty Images

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When so many fringe players have been given a rare opportunity to work their way into a Test team, you would assume they would work themselves into a frenzy. Instead so many of those who ran onto the field appeared lethargic, were error-prone, and often seemed not to know what they were doing.

There were also some confounding moments, such as in the 30th minute when Henry Speight, playing in the centres, kicked the loose ball through towards the US line. Here was the chance to really put pressure on the US defenders, but instead Speight just went through the motions, merrily jogging towards his opponent. Speight then got bounced off in a half-hearted tackle.

A minute later the Eagles scored.

As diabolical was the Wallabies poor kicking game and continual stupid moments when, instead of offloading, they haphazardly took on the US defence and got smashed. They also too often drifted across field in attack.

At half-time, the Wallabies were justifiably only four points ahead. Then on came the reserves, the most important change being Will Genia taking over from Nick Phipps at the scrum base. Phipps failed to take control in the first half, and so the Wallabies often produced loose, disjointed play.

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Genia tightened it up, attacked the line far more, and soon had the Wallabies playing well-constructed phase football. With that far tighter structure, and better work at the breakdown, the tries began to come. A red-face moment was averted.

Of the Australian players who started the Test, only two grabbed the opportunity. Sean McMahon was the match standout, providing consistency and momentum when so many of his fellow forwards floundered. The work-rate of Kane Douglas was also of a high standard - being used extensively as the team's main lineout catcher. Cheika was clearly using this game to check Douglas's level of fitness, and the lock was still bounding around the field late in the game; a good World Cup sign.

The other positive of the game was that Bernard Foley's goal kicking has picked up even if his general play remains wonky.

But prop Scott Sio, who is being trained to be the team's third hooker, had one chance to throw in the lineout in the game. He threw; the Wallabies lost the lineout.

It was that type of game: an important PR exercise; but as a final hit out before the World Cup it was a long way short of being satisfying.