An MRI scan on Sunday morning revealed the Leinster winger has a 50-50 chance of returning in time for the Pool D clash with Georgia in Bordeaux and O'Sullivan will use the match as the deadline for his rehabilitation.
Should Horgan - one of Ireland's most dangerous strike runners - fail to recover in what would be a sickening blow for player and country, Tommy Bowe will be named his replacement for the remainder of the competition.
"The injury will probably keep Shane out for four weeks, depending on his powers of recovery," said O'Sullivan.
"On that basis he has a chance of playing in the second Pool game against Georgia and it's just a case of seeing how quickly he heels.
"That's the target and we'd like to keep to it but it's still five weeks away yet.
"We may make the decision before that if we get more information. Shane's a good healer and we believe he'll make it.
"Tommy will stay with the squad until we get more data on Shane, but we're doing the best we can to keep our options open.
"We'll know more on how Shane's progressing over the next week or two.
"But if it becomes evident during that time that he won't make it, then we won't wait to make a decision."
Horgan's swift recovery two weeks ahead of schedule from the knee injury that forced him to miss the start of the Six Nations will have influenced O'Sullivan's decision.
So too will have his value to the team as the 29-year-old provides crucial midfield cover as well a dynamic edge in attack.
Rules state an injured player can be replaced at any point in the World Cup, but the change is permanent hence O'Sullivan's willingness to give him every chance to recover.
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 in a straight shoot-out for a ticket to France and Carney, who converted from rugby league five months ago, has edged the contest.