Underhill was overlooked altogether for the 30-28 defeat at Optus Stadium in Perth but is winning the race to be selected at openside flanker ahead of Lewis Ludlam and Jack Willis.
The 25-year-old was a star of the 2019 World Cup but since Japan he has been limited to winning only 13 caps due to concussion, COVID, illness and injury, making just one appearance in the recent Six Nations.
Now he is poised to lead the defensive stand at Suncorp Stadium as England look to take the series to a decider, with Ludlam expected to continue on the bench.
"Sam's work rate on and off the ball are excellent. He's a really good defender, particularly with his chop tackles," defence coach Anthony Seibold, who joined fellow Australian Eddie Jones' coaching team in the autumn, said.
"He's played at a high level. I'm thinking of New Zealand at Twickenham in 2018 and, watching his performances at the World Cup from afar, he was a standout for me."
Teenage sensation Henry Arundell is also in serious contention to replace Joe Cokanasiga on the left wing following his stunning seven-minute try-scoring cameo in the first Test.
It was hoped that Jonny May would be available, but he has yet to train fully as he continues his recovery from COVID and now faces a battle to be fit for the climax to the series in Sydney.
Six years ago England produced a remarkable act of defiance in the second Test in Melbourne to take an unassailable lead in a series they went on to win 3-0, and it is felt that a similar effort will be needed to subdue the Wallabies on Saturday.
"You look at our leaders like Courtney Lawes, Owen Farrell and Ellis Genge - you want to go to war with those guys. You do," former South Sydney and Brisbane NRL coach Seibold said.
"I know Owen spoke about how this is an opportunity for us. I do think we've got some guys in our group who are up for this and that's the exciting thing."
Veteran prop Mako Vunipola has called on England to empty the tanks at a venue where Australia have won their last 10 Tests.
"It's time for us to show what we're about as a team. We've got nowhere to hide," Vunipola said.
"We've got our backs against the wall and now we've got to go out there and put our best foot forward."
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