South Africa's opening batsman Dean Elgar was diagnosed with a concussion after he suffered a blow to the head in the third Test against India on Monday.
The player will not be allowed to return to action for at least six days, the team's management said.
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Elgar misjudged a bouncer from fast bowler Umesh Yadav and failed to duck in time as the ball crashed into his helmet above the ear, forcing the dazed 32-year-old to retire hurt.
"The opening batsman will be closely monitored over the next 24 hours, followed by reassessment every 24 hours until signs and symptoms resolve," the South African team said in a statement.
"The soonest he can return to play is in six days' time in keeping with the standard concussion management protocol."
Theunis de Bruyn was named as a concussion substitute under the new rules and the 27-year-old was unbeaten on 30 at stumps, with the Proteas floundering on 8-132, still 203 runs behind India after being asked to follow on.
India lead the three-match series 2-0 and, with two days left in the test, Virat Kohli's side need two wickets to clinch a series whitewash and consolidate their place on top of the World Test Championship table.
Hope he is alright. Nt a gud sight, being a cricket fan myself.— Ayush Tayal (@monty7tayal) October 21, 2019
A concussion substitution for Dean Elgar. That was nasty from Yadav— Arpit Sharma (@ArpitSh05858008) October 21, 2019
That's very nasty blow onto Dean Elgar's helmet, he isn't feeling well, don't like to see any player going down that way, Umpires have called for the Tea Break so he'll have time to recover #INDvSA pic.twitter.com/PxdKZwmXhY— Abhijeet ♞ (@TheYorkerBall) October 21, 2019
Never a nice sight to see someone get hit on the helmet. It looked nasty. Hope Dean Elgar is okay 🙏.— Cricerz 🏏 (@Cricerzfantasy1) October 21, 2019
Lovely to see the Indian players gather around to check if he's okay. Sportsmanship wins.#INDvSA #Cricket pic.twitter.com/F6SrRU98CF
Day of firsts
India's wicketkeeper Wriddhiman Saha suffered a finger injury in the final session and Rishabh Pant took over.
According to new rules that allowed a concussion sub, a team is also allowed to replace a wicketkeeper.
Shahbaz Nadeem, making his debut at the age of 30, also claimed two wickets with his left-arm spin and said it was a "dream" to play for the national side after taking 424 first-class scalps in Indian domestic competition.
Zubayr Hamza, playing just his second Test, was happy to score his maiden Test half-century to offer some resistance during his 91-run fourth-wicket stand with Temba Bavuma, who made 32.
"But also disappointed in the same breath, not having gone on to make use of that opportunity,” Hamza said.
“And the first time that I've been out twice in the same day, so it's been a tough day. Really disappointing for the side."
Opening batsman Rohit Sharma put India in command of the match with his maiden Test double century on Sunday. He made 212.
India won the first two Tests by big margins in Visakhapatnam and Pune.