Best was admitted to Pellegrin hospital in Bordeaux last night after developing a loss of sensation down his right side along with a mild headache and a difficulty in speaking. Continuing tests are being carried out in order to give a clear diagnosis on his condition.
Unfortunately due to these symptoms he will be unable to take any further part in the World Cup and will be returning home when he is given medical clearance to do so.
The 29-year-old's place on the replacements bench for Sunday's game against Argentina has been taken by his Ulster team-mate Bryan Young, while Munster prop Tony Buckley has also been called up to the Irish squad as cover.
Hooker Rory Best, Simon's younger brother, will still undergo a fitness test on a thumb injury this afternoon to try and regain his place in the Irish team for this weekend's Paris encounter.
O'Sullivan said: "Rory's a bit upset but he knows that Simon is in the best possible hands. Everythingâ00s being done for him. At this stage, itâ00s just waiting for them to find out the problem is the worst thing. But Rory's fine. We spoke to him earlier and he'll be going training after lunch.
â0I wouldn't speculate on anything and I don't think anyone should. It would be wrong for everybody, including his family. We don't have a diagnosis. Hopefully, itâ00s something very small. Please God, it isnâ00t as bad as it could be. Our thoughts are with him. He's our priority at the moment and we've all got our fingers crossed for him.
"Currently, his condition is that he's been moved for further tests. They (the doctors) won't speculate. I'm sure they have opinions but you can't expect them to speculate on these things."
Asked about what the reaction has been in the Irish camp to Best's hospitalisation, he added: "We're all worried about Simon. Heâ00s been around for a long time and heâ00s one of these quietly-spoken guys that everybody loves. So itâ00s a shock for everybody that something like this can happen.
"We've got to deal with this first. Weâ00ve got to also deal with the fact that we've to get up for a game on Sunday. Weâ00re going to training after lunch. Rory still has to clear a fitness test, thatâ00s the first hurdle. Weâ00ll get that out of the way this afternoon. Then we'll talk to Rory and then sit down and have a think about it.
â0Simon's wife is en route here at the moment. His dad will be here tomorrow. He will be going home at some stage so obviously, we want family travelling with him."
Best felt the symptoms come on him all off a sudden yesterday afternoon, when the Irish squad were enjoying some time off training. O'Sullivan explained: "He was in downtown Bordeaux with his good friend Paddy Wallace, just strolling around the city centre, when he felt this coming on. He called our team doctor (Gary O'Driscoll), we got him back to our hotel immediately and then we moved him to the hospital straight away. Heâ00s been in hospital since."
"There's no time frame. Gary Oâ00Driscoll has been with him since this happened and he's keeping us updated. But now that he's moved hospital I suspect that there wonâ00t be any news until theyâ00ve done further tests. That could take a number of hours, I donâ00t know.
"Paddy just described the symptoms as I described them to you - pins and needles down his right side, loss of feeling, headaches and loss of speech. They were obviously worried straight away, contacted the doctor and that put the wheels in motion. Some of our World Cup liaison guys were able to swing in and pick him up. The transfer was pretty swift. It was around tea-time and very sudden."
Commenting on how Best being taken ill has affected the Irish camp's preparations for the game against Argentina, O'Sullivan conceded: "We've had to move on that obviously. It's evident that he wonâ00t take any more part in the tournament so weâ00ve got Tony Buckley en route from Munster to fill that spot.
â0We have an issue that we didn't expect to have with one of our friends, one of our family. We have to get on with our preparation because the game is going to go ahead regardless. Itâ00s just about getting that balance right in our heads. We're training this afternoon so we've to go out and get that part of the job done. Hopefully, when we come back from training we'll have better news from the hospital."