Chris Henry suffered 'a temporary blockage of a small blood vessel in his brain' on the morning of Ireland's clash with the Springboks, team bosses have confirmed.
Ireland medics initially diagnosed a virus on the morning of the November 8 clash before suspecting a severe migraine, but specialists have since determined the Ulster flanker suffered a more unusual condition.
The 30-year-old missed Ireland's 29-15 Springboks victory and now 'needs further investigation and specialist opinion,' according to an Irish Rugby Football Union (IRFU) statement.
Chris Henry suffered some weakness on the morning of the South Africa game and a viral illness was suspected, he recovered quickly and the initial diagnosis was of severe migraine.
Further tests however have shown he suffered a temporary blockage of a small blood vessel in his brain.
He is at home now and is well but needs further investigation and specialist opinion.
England winger Anthony Watson has admitted that his own mother hates watching him play, preferring snooker and tennis.
England head coach Stuart Lancaster has given his backing to winger Anthony Watson ahead of his first international start.
Watson, 20, made his debut in the defeat to Australia on Saturday, but gets a place in the side from kick-off against South Africa this weekend.
"Anthony has been pushing hard for a while," said Lancaster. "He has got all the physical attributes you need. He's tall, good in the air, has pace and good footwork. Now his experience will be tested.
"His chance has come because of the injury to Roko, and his form for Bath this season, but also because of the conversation I had with him before the New Zealand match.
"It would have a big step for a 20-year-old to go into a match against the world champions from the start but what came across was how confident he was and how prepared he was. He is definitely ready."
Ireland fly-half Jonathan Sexton has been nominated for the 2014 International Rugby Board world player of the year award.
Sexton, who played an instrumental role when Ireland beat South Africa last weekend, is among five nominations announced by the IRB.
The 29-year-old has been joined on the shortlist by South Africa full-back Willie le Roux and Springboks number eight Duane Vermeulen, and New Zealand wing Julian Savea and All Blacks lock Brodie Retallick.
England head coach Stuart Lancaster is hopeful that forwards Courtney Lawes and Dylan Hartley will be fit to face South Africa on Saturday.
Both players suffered head injuries against New Zealand at the weekend, but Lancaster is optimistic that they will pass the mandatory concussion tests.
"Courtney is symptom-free so hopefully he will be okay. Dylan has also got bit of a head knock and is in the same situation," Lancaster said.
"They need to train on Thursday to be ready. They will do a cog Test on Thursday morning which they have to pass to train.
"We will have provisions in place should they fail that, but we are optimistic will be available."
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England centre Manu Tuilagi has been ruled out of next month's autumn series because of a groin injury, his club Leicester have announced.
Tuilagi limped off after 15 minutes of Saturday's 25-18 Champions Cup victory over Ulster and will now miss the Tests against New Zealand, South Africa, Samoa and Australia.
"I'd be very surprised if Manu plays any part in the autumn. He'll clearly be missing for a few weeks as a minimum," Leicester director of rugby Richard Cockerill said.
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Former rugby player Martin Offiah has said he doesn't see any point in "putting Ben Flower in jail" after the Wigan player twice punched an opponent during the Super League Grand Final.
Flower punched the St Helen's half-back Lance Hohaia in the face twice just a few minutes into Saturday's final. The first strike floored him, while the second came as he lay passed out on the ground.
Wigan legend Offiah said he saw no role for the police in the matter: