England have named an unchanged team for Saturday's World Cup semi-final against Tonga but stand-off Kevin Brown will have to pass a series of tests before he can play.
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He was knocked out during his team's 36-6 win over Papua New Guinea on Sunday and withdrawn at half-time.
If he is not passed fit to play Gareth Widdop is likely to move from full-back, with Jonny Lomax replacing him.
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Fit-again Lomax and Scott Taylor have been brought into the reserve list.
They have replaced Stefan Ratchford and George Williams - and could yet figure in the final 17 if Brown fails his assessment or any players pick up injuries in training.
England are hoping to reach their first World Cup final since 1995 and the absence of Brown would be a blow to a side that has looked more solid and organised since he was brought in against France at the end of the group stage.Brown was knocked out after colliding with an opponent 25 minutes into the quarter-final win in Melbourne but replays were not working in the stadium and the England team were unaware of the fact that he had been momentarily knocked unconscious.
An England player who was not in the matchday squad then spotted the incident on social media and informed the doctor, who withdrew Brown at half-time.
England coach Wayne Bennett said after Sunday's match that the concussion was rated category one, the lowest grading, and he was confident Brown would be cleared to play.
If Brown is passed fit then he will once again partner Luke Gale in the halves, with Widdop able to continue his impressive form at full-back.
Two changes for Tonga
England face a Tonga side that are unbeaten in the tournament - although they were given a real scare during their 24-22 quarter-final win over Lebanon on Saturday.
In many ways the surprise package of the competition, Tonga are unbeaten and claimed the notable scalp of New Zealand in the group stages - a result that placed them in the same half of the draw as England.