D'Arcy limped off in the first half of his side's opening championship fixture against Italy and, despite a spell in a specialist rehabilitation centre in Spala, Poland, he has not figured in Ireland's Grand Slam bid since.
A run-out for Leinster on the weekend confirmed the 25-year-old is not yet ready to make his Test match return and he has been ruled out of Saturday's mouth-watering clash with France at Lansdowne Road.
But O'Sullivan's assessment that D'Arcy might not be fit for the potential Grand Slam decider against Wales on March 19 is a more significant blow, given time is running out to stake his claim for a Lions berth.
"I'm worried about Gordon's availability for the Wales match because his recovery has slowed down so dramatically. The situation is a concern," said O'Sullivan.
"We were confident last week that he would be fit for France and we thought he would be fine once he got onto the pitch for Leinster. But he didn't have as much power as he wanted and at that juncture we knew he wasn't right.
"He had some discomfort in his hamstring and underwent a scan which revealed some residual scarring that hasn't cleared up. That's the reason he hasn't got full power.
"He's 95 per cent there but that last five per cent is taking a lot longer to heal than expected.
"He hasn't improved much over the last week. He can run but not at full speed. It's a Test match so it's not as if we can throw him in and see what happens."