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 stoke, and today 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 tomorrow'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.''

In other news, Andrew Trimble has been withdrawn from Ireland's bench for tomorrow's World Cup clash with Argentina.

Trimble suffered a fractured finger in the Pool D opener against Namibia and although he returned for the 25-3 defeat by France, the wound has become infected.

The Ulster winger's place among the substitutes goes to Gavin Duffy of Connacht.

Ireland coach Eddie O'Sullivan said: ``Andrew is out of tomorrow's game. He had a finger injury against Namibia and lost the nail.

``It's been treated all along but has picked up an infection in the last 24 hours and hasn't cleared up.''