"On closer examination it was found that a bony growth had developed in the ankle. These things can happen, so they thought the best thing is to get it removed.
"It was the sensible thing to do - it could be left and it would keep happening to him, that he would get worse as the week goes on. It's a small operation and he will be out for a few weeks."
While Leinster flanker Shane Jennings continues to be sidelined by a fractured thumb, O'Sullivan also issued an update on the fitness of Gordon D'Arcy, David Wallace and Ronan O'Gara, who have each sat out some training at this week's camp in Limerick due to minor knocks.
"Gordon has a slight calf strain. We will give him a few more days but he is ready to go. David Wallace has a chronic ankle problem. It reared up again last week and we rested him this week, he stayed out of contact stuff.
"Ronan tweaked his knee recently. He's been working hard all week and we will give him a few days' rest now but he is okay."