The 25-year-old Leinster centre - who has played just twelve times for both province and country this season, with his one Ireland Test against Italy - was ruled out of contention by O'Sullivan for Saturday's clash with France.
"Gordon is not fit. He played for Leinster on Sunday and we were confident that if he got through the game he would be okay," admitted the Ireland coach.
"He had some discomfort in his hamstring and underwent a scan yesterday which revealed some residual scarring that hasn't cleared up.
"He's 95% there but that last 5% is taking a lot longer to heal than expected. He hasn't improved much over the last week. He can run and sprint but can't open up to full speed."