Smith suffered delayed concussion symptoms after being hit on his neck by a 92mph bouncer from England fast bowler Jofra Archer.
Leeds: England captain Joe Root wants his side to make the most of Australia star batsman Steve Smith's enforced absence from the third Ashes Test at Headingley starting Thursday as the hosts try to level the five-match series at 1-1.
Smith suffered delayed concussion symptoms after being hit on his neck by a 92mph bouncer from England fast bowler Jofra Archer during the drawn second Test at Lord's and has since been ruled out of action in Leeds.
His absence is a setback for Australia, while England have a slight doubt over opener Jason Roy after he suffered a suspected concussion batting in the nets.
Smith marked his return to Test cricket after a 12-month ball-tampering ban with innings of 144 and 142 during Australia's 251-run win in the first Test at Edgbaston that put his side 1-0 up.
He also made 92, having retired hurt after being hit when on 80, at Lord's where Marnus Labuschagne, Test cricket's first concussion substitute, made a brave fifty in the second innings after being hit flush on the grille of his helmet by Archer.
England too have been without a key man this series after James Anderson, their all-time leading wicket-taker, broke down with a calf injury having bowled just four overs at Edgbaston.
"They will be huge shoes to fill," Root said of Australia being without Smith.
"It's very unfortunate on their part but we had the challenge of Jimmy going down four overs into a Test match and had to manage that without a replacement.
"You get these swings within a big series like this every now and again and when you get your opportunity you've got to jump on it.
"There was a lot of concern in our dressing room for his health but it's great to see he's up and about and is fine."
World Cup-winning fast bowler Archer struck several telling blows on a Test debut that yielded five wickets.
"He's dealing with it pretty well, I think," said Root of the hype surrounding Archer.
"He's just a very relaxed character, unfazed by anything. He's one of those guys who seems to be born for these occasions."
Australia also have plenty of fast-bowling firepower at their disposal in Pat Cummins, Josh Hazlewood and Mitchell Starc with James Pattinson also waiting in the wings.
But coach Justin Langer insisted his side would not let talk of a 'bouncer battle' sidetrack them from their goal of becoming the first Australia side in 18 years to win an Ashes series in England.
"We know what our plans are to beat England," Langer said. "It's not an ego game.
"We're here to win the Test match, not to see how many bruises we can give."
Smith and England opener Rory Burns apart, both top-orders have struggled in the face of some quality fast bowling.
But England have stuck by Roy, even though the World Cup-winning opener averages just 10 in four innings this series.
Despite his blow in the nets, Roy is expected to be fit to play, although Ollie Pope has been summoned as cover.
Roy has mainly been a middle-order batsman in first-class cricket and England coach Trevor Bayliss said, ideally, that is where he would bat in a Test line-up.
But Root said Roy could yet be a successful Test opener. "I feel Jason can have a real big impact at the start of an innings," said Root, England's leading batsman.
Australia captain Tim Paine said: "Opening the batting in Test cricket in England is difficult. I know our guys are struggling a little bit as well to get the runs they would like.
"It's international cricket, it's tough," the wicketkeeper added.