Great Britain's stars admit they will need to be more ruthless if they are to reach the Gillette Tri-Nations final.
Paul Wellens scored Great Britain's first try on 74 minutes
The Lions began their campaign with a 18-14 loss to New Zealand at Jade Stadium in Christchurch on Saturday.
"We had enough fire and passion but not enough smarts," said skipper Jamie Peacock, whose team play Australia next. "We've got to be more clinical."
Full-back Paul Wellens added: "We have got to do better with the football in the opposition's third of the field."
Britain only managed to breach the New Zealand defence in the last six minutes, Wellens crossing before second rower Gareth Ellis touched down for his first try for the Lions.
"We are very disappointed we haven't won the game," said Wellens. "But we are very confident we can improve on that and give the Aussies a good run for their money next week."
Lions coach Brian Noble refused to be too downhearted, insisting his team had been a little rusty.
It was definitely knocked out when I was over the line so in my opinion it was a penalty try or at least our ball
"We will probably kick ourselves that we didn't execute as well as we can, but there are a lot of positives we can take out of that performance," he said.
"I think we will be a bit more fluid next week. We lacked a bit of patience, but we will learn from that and get better."
Noble was also reluctant to make too much of an issue of McGuire's disallowed try at the start of the second half.
But the Leeds stand-off felt the Lions should have been awarded a penalty try after the ball was knocked from his grasp by Brent Webb as he dived for the line.
"It's a bit disappointing, but stuff like that happens in the game," he said.
"It was definitely knocked out when I was over the line, so in my opinion it was a penalty try - or at least our ball."