'Kind of helpless': Steve Smith scare left Aussie hero 'sick'

Marnus Labuschagne copped a nasty Jofra Archer bouncer to the grille but it didn't rattle Australia's super sub nearly as much as the blow to Steve Smith's neck.

Labuschagne stepped up on day five of the second Ashes Test in unprecedented circumstances, facing more balls than any other teammate to ensure the tourists salvaged a draw.

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The 25-year-old was in the nets facing James Pattinson and Mitchell Starc when informed by Tim Paine that he'd become international cricket's first concussion substitute.

Smith retired hurt after being struck by a delivery from Archer on day four at Lord's then returned to bat after passing concussion tests, only to wake up on Sunday with a headache.

Labuschagne, the closest thing Australia had to a like-for-like replacement for the world's best batsman, started shakily.

The first legal delivery he faced, a searing short ball from Archer, thumped the helmet.

"You get up and try and act cool ... it got me quite flush in the grille, so it took most of the blow," the right-hander said.

"Being out there is probably less nerve racking than being off the field. When he (Smith) got hit, I had a bit of a sick feeling. You're kind of helpless ... little bit of a shock to the system.

"But when you're out there, you're in the contest and the adrenaline is rushing."

Labuschagne was assessed for signs of concussion by team doctor Richard Saw, who was kept busy throughout Archer's bumper barrages at Lord's.

"You just want to stay calm and make sure you answer his questions properly," he said.

"I was like 'I know where I am, I'm good, I'm good. Just get off the field' but there's a process,

"I was feeling fine, I just jumped the gun a bit on the questions."

Labuschagne, who first shot to fame as the substitute fielder who clutched a classic catch at short leg during a Gabba Test in 2014, is now preparing to face England when the Ashes continue in Leeds on Thursday.

Smith is expected to miss the third Test as he recovers from concussion but even if the superstar is passed fit, Labuschagne's 59 will ensure he remains in the mix.

Selectors may be tempted to promote Usman Khawaja to open then replace Cameron Bancroft with Labuschagne.

"You just want to be ready (to play)," Labuschagne said.

"Especially with the new rule ... one minute, you're hitting balls in the nets then all of a sudden you're playing. It can happen very quickly."