O'Sullivan was forced to withdraw 2004 Player of the tournament Gordon D'Arcy from his original squad this morning, and with captain Brian O'Driscoll suffering a similar fate with another hamstring strain - the Ireland coach feels their absence is a "fairly serious setback".
''I think we have managed to hang onto one of them - O'Driscoll and D'Arcy - for every game in the last five but now we've got to deal with both out," said the Cork man.
''It's not what we were looking for but at the same time we knew this was a possibility almost a week ago and we've been working around it.
Set for his fortieth game in charge of the men-in-green, O'Sullivan ruled out any 'mind games' as regards his delayed selection, adding: ''At the start of the week there was a chance with the injuries, and we had to play it out. It would be foolish man who ruled them out on Monday morning - to find they were on the pitch today.
''It was obvious Brian wasn't as far on as Gordon by Thursday and we said we would give it until Friday. Gordon wasn't far away but considering this is only our second game of the championship and there are still three to go after that. It was sensible caution.''