Martyn Williams believes the Lions will "definitely" beat Australia in the winner-takes-all third Test on Saturday if skipper James Horwill is banned.
Lock Horwill is awaiting the result of an International Rugby Board appeal against a citing commission decision to clear him of stamping.
Former Wales and Lions flanker Williams thinks the decision could decide the series, which is tied 1-1.
"If he's out of the Australian side, they really struggle," said Williams.
Horwill was cited after the British and Irish Lions' 23-21 opening Test victory over Australia in Brisbane on 22 June.
Television pictures showed his foot coming into contact with Alun Wyn Jones's face while the Lions second row was lying on the floor.
But judicial officer Nigel Hampton QC decided he could not be certain of an intentional act of stamping.
That handed Horwill a reprieve which allowed him to lead Australia to a 16-15 win in the second Test in Melbourne on 29 June.
The IRB's appeal against the decision was heard on Monday, 1 July with judicial officer Graeme Mew, who is based in Canada, expected to announce his decision on the same day (BST).
If the verdict is overturned, it is likely Horwill will be banned and miss the third Test in Sydney on 6 July.
Williams, a Lions tourist in 2001, 2005 and 2009, thinks the outcome will be a decisive moment in the series.
"The key thing this week is if James Horwill gets banned or not," he said.
"They've [Australia] got so many injuries and haven't got strength in depth, so I think that's the key factor.
"If he's missing I think the Lions will definitely win."
who is ruled out by a hamstring injury suffered in the second Test.
And Williams believes the absence of his former Cardiff Blues team-mate is a major handicap as the Lions bid for their first Test series win since 1997.
"He [Warburton] was unbelievable at the contact area and in some ways kept the Lions within touching distance of Australia," said Williams, who won 100 Wales caps.
"People have questioned him from the start of the tour, but as the tour has progressed he's got better and better.
"It's a big blow not just for Sam himself but for the Lions in general.
"There's no doubt it's going to be [Brian] O'Driscoll as captain.
"And in the seven spot whether they go with Justin Tipuric or Sean O'Brien is going to be fascinating."