Steve Smith was ruled out of the final day of the second Ashes Test with concussion after the Australia batsman was struck in the neck by a 92mph Jofra Archer bouncer on day four.
Smith passed on and off-field concussion tests after being hit, returning to complete his innings after initially retiring hurt but looking shaken and unsteady during his brief return to the crease.
The 30-year-old was carefully assessed by medical staff through the night but reported "a bit of a headache and a feeling of grogginess" on waking.
Subsequent concussion testing showed a deterioration in Smith's responses and he was withdrawn from the final day as a result.
All-rounder Marnus Labuschagne was approved as Test cricket's first concussion substitute by match referee Ranjan Madugalle, meaning he is available to bat and bowl for the remainder of the game.
Smith will go for a precautionary scan on his neck and his chances of playing in next week's third Test at Headingley appear remote given the return-to-play protocols governing head injuries.
"As part of the Cricket Australia concussion protocol, repeat concussion testing of Steve Smith was performed this morning and demonstrated some deterioration from his testing, which is consistent with the emergence of the symptoms he was reporting," said a Cricket Australia statement.
"Cricket Australia statistics show that 30 per cent of concussions in Australian cricket are delayed.
"It is not uncommon for players to pass their tests and feel well on the day of an injury and then display symptoms 24 to 48 hours later.
"In terms of Steve's availability for the third Test, this will be considered over the coming days but the short turnaround to the next Test is not in his favour. Steve's fitness will be assessed on an ongoing basis."
