Horgan was named on the bench for yesterday's 31-21 defeat by Scotland but twisted a knee during the pre-match warm-up and was immediately withdrawn.

Ireland coach Eddie O'Sullivan admitted after the game he was concerned by the injury with Horgan, one of the side's most dangerous strike runners, a shoo-in at right wing.

The Leinster back, who also provides crucial midfield cover, went for an MRI scan this morning which revealed a grade two ligament tear.

If his rehabilitation progresses as expected, he will be available for Ireland's second Pool D clash against the Georgians in Chaban-Delmas.

Tommy Bowe will travel with the 30-strong squad to France and if Horgan fails to recover in time to face Georgia, the Ulster winger will be drafted in as his replacement for the remainder of tournament.

O'Sullivan said: 'Shane's injury means a four-week lay-off, depending on his healing powers.

'That puts him in with a chance of making the second Pool game against Georgia.

'Whether he makes that deadline we have set him is another matter but we're giving him every chance. He's a good healer and we believe he can make that.

'Bowe will stay with the squad until we get more data on Shane. Keeping our options open is the best we can do at the moment.

'Shane is a 50-50 chance to make the Georgia match. We'll know more over the next couple of weeks so we may make a decision sooner.'

Horgan's inclusion means just one place remained open in the back three and O'Sullivan has opted for Munster's Brian Carney over Bowe.

Both wingers featured at Murrayfield yesterday in a straight shoot-out for a ticket to France but Carney, who converted from rugby league five months ago, has edged the contest.

Bowe, however, could yet feature at the World Cup if Horgan fails to overcome his ligament strain.