Simon Best has been diagnosed as having an irregular heartbeat, according to Ireland coach Eddie O'Sullivan.
Best was admitted into hospital on Wednesday night complaining of having difficulty speaking, a headache and a loss of feeling down his right side.
Specialists eliminated neurological problems as the source of his condition, easing concerns the Ulster prop had suffered a stroke, and on Saturday confirmed the symptoms were cardiac-related.
The health scare has ended Best's World Cup with O'Sullivan revealing the 29-year-old is likely to return home on Tuesday.
He said: 'Simon has been diagnosed as having an irregular heartbeat, the cause of which we don't know and may never know.
'He's had another scan on his heart and has been given the all clear.
'As soon as the doctors are happy for him to travel back home he will be released. But that probably won't happen until Tuesday.'
Brian O'Driscoll insisted Best's absence from Sunday's World Cup clash with Argentina - the front row had been named on the bench - would spur Ireland on at the Parc des Princes.
He said: 'When something like that happens to someone in the squad just out of the blue, everyone feels it.
'There's a big element of relief that he came through and it wasn't more serious.
'It was hugely unfortunate for him and while we must get on with things, he's still very much in our thoughts.
'But these things always do have a galvanising effect.
'What happened last week in the French press had a galvanising effect, although granted we didn't produce the performance we felt the tightness of the squad deserved.
'But we're a very close squad. Simon's been with us for a while and has been an important part of our 22 for the last four or five Six Nations.
'For him not to be there feels like someone is missing.'