D'Arcy, whose fitness struggles this season have not helped his claims for a Lions tour spot, was ruled out of Saturday's 26-19 defeat by France with the hamstring injury he suffered in the opener against Italy.
He has played just over 20 minutes of Six Nations rugby this season.
The 25-year-old Leinster centre visited a specialist rehabilitation centre in Spala, Poland three weeks ago, but his recovery has slowed since and O'Sullivan is doubtful he will make the trip to Cardiff.
"I don't think Gordon is going to make the Wales match the way things are going. There has been uncertainty over his injury for the last few weeks and it's not looking good," he said.
D'Arcy was voted last year's Six Nations player of the tournament and his presence relieves much of the creative burden shouldered by skipper Brian O'Driscoll.
Shane Horgan has filled in for D'Arcy in the past but also missed out against France and his thumb injury will be reassessed before Tuesday's team announcement.