Steve Smith Ruled Out Of Test With Concussion

Steve Smith has become international cricket's first concussion substitute and is in immense doubt for the third Ashes Test.

Smith has been diagnosed with concussion and will sit out the rest of the second Test, while he will soon undergo a precautionary neck scan.

Marnus Labuschagne has replaced Smith in Australia's XI as a concussion substitute, as per a recent rule change introduced by the International Cricket Council.

The third Test starts on Thursday in Leeds, leaving Smith little time to prove he has recovered from a nasty knock to the neck.

Smith was struck by a Jofra Archer bouncer while batting on day four at Lord's, where he retired hurt before resuming his innings after passing various concussion tests.

The former skipper slept well on Saturday night but woke with a headache and had also felt dizzy, drowsy and groggy.

READ MORE: Steve Smith Cops Sickening Blow To The Neck In Ashes Test

Photo: Getty Images.

Sunday morning's CogSport exam was another red flag for the 30-year-old. The computer program tests an athlete's reaction time, attention and memory then compares data with previous baseline results.

Cricket Australia's concussion and head trauma policy does not dictate the gifted batsman must sit out a specific period of time beyond the initial 24 hours.

However, it is hard to envisage how the superstar will be able to complete a gradual return to physical activity to a point where he is passed fit for the next Test.

Team doctor Richard Saw, having conducted Smith's concussion tests throughout the weekend, will assess the right-hander regularly this week.

Smith is the first cricketer to be substituted out of an international match, a law that was introduced for the start of the Ashes.

Australia's decision to let Smith bat on day four after being hit will be heavily scrutinised.

Smith passed all his tests on Saturday. However, it is not unusual for concussion symptoms to appear 24-48 hours after a head knock.

Photo: Getty Images.

Coach Justin Langer, who famously wanted to bat in a Test against South Africa in 2006 despite being hospitalised with concussion, defended the call to let Smith bat.

"These are like my sons alright, so you're never going to put them in harm's way," Langer said after day four.

"I was saying 'mate, are you sure you're ok?' ... I asked him behind closed doors two or three times. I asked him in front of the group.

"He just kept going 'all good, all good coach. I'm ready ... I can't get up on the honours' board unless I'm out (there) batting'.

"What else do you do? The medicos cleared him."

Smith was hit on the elbow, forearm and neck by Archer during Saturday's violent spell of short stuff.

The third and most-brutal strike prompted the shocked crowd to fall silent as Smith slumped to the ground.

Smith was back on his feet soon after and keen to keep batting but left the field at the insistence of Dr Saw, who wanted to conduct a thorough concussion assessment in the rooms.

Broadcasters initially didn't show a replay of the blow, such was the level of concern after an incident that dredged up memories of Phillip Hughes' tragic death in 2014.

"There's obviously some pretty rough memories of a blow like that," Langer said.