The pair - tipped by many to be the first choice British and Irish Lions centre partnership in New Zealand - are now in some doubt for next week's second round of RBS 6 Nations games, when the Irish face Scotland.
Coach O'Sullivan said on BBC2: "Both of the them have the same injury - both have picked up hamstrings.
"Gordon when he was running in full flight and Brian in a ruck when he got hit from behind.
"Gordon isn't good and I think will be under pressure for next weekend.
Brian, I don't know yet. It doesn't look great to be honest.
"He was tired as well so it's hard to know how much damage there is, but the doctor and the physio are looking at it at the moment."
Ireland, many people's favourites for the RBS 6 Nations crown, were made to struggle in Rome by an Italy side who led 9-8 early in the second half.
"It was a hell of a tough game," added O'Sullivan. "We struggled in the first half because we hadn't the football.
"I think we had our first attack in the Italian half after 22 minutes. We had a good return, with three first-half possessions in their half and we scored twice.
"The second half was about spending more time in their half."
O'Sullivan was determined that the home side should take credit for their performance.
"Italy played really well," he added. "They handled the ball well in terms of kicking it, if that's not an oxymoron.
"We struggled with the ball and couldn't put accuracy and length on it. It hurt us in the first half when we needed to get out of trouble, but we've stuck to our task in the second half and brought them down.
"We said before the game that it might take until 10 minutes from the end for this game to be won, and that's how it turned out.
"So no real surprises there, but a tough week ahead of us now with Scotland."