Dublin - Ireland's Chris Henry missed the recent win over the Springboks with a brain injury, rather than a migraine, the Irish Rugby Football Union have announced on Monday.
Team medical staff initially diagnosed a virus on the morning of the November 8 match before suspecting a severe migraine.
However, specialists have since determined that "a temporary blockage of a small blood vessel in his brain" forced 30-year-old Ulster flanker Henry to miss Ireland's surprise 29-15 victory over the Springboks.
"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," read the IRFU statement.
"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," the statement added.
Henry helped Ireland win last season's Six Nations after replacing injured Leinster back-row Sean O'Brien.
With O'Brien still sidelined, Henry had been in line to continue to star in a loose forward trio also featuring Jamie Heaslip and Peter O'Mahony.
Henry's injury led to a late call-up for Rhys Ruddock, who then scored his first Test try for Ireland against the Springboks.
Ruddock, son of former Wales coach Mike Ruddock, could now be selected again for Saturday's match against Australia after a much-changed side saw off second-tier Georgia 49-7 in Dublin on Sunday.