O'Brien was expected to be out for two weeks after incurring bone bruising in last weekend's Amlin Challenge Cup final victory over Stade Francais. But Lions forwards coach Graham Rowntree has revealed that the injury is not as bad as initially feared and that the 26-year-old Ireland back row could be in contention for selection against Ulster at the RDS.
"Sean is getting a lot better and has improved quickly over the last 48 hours and with regards to Saturday, watch this space," Rowntree said.
O'Brien will miss out on the Lions' opening fixture against the Barbarians in Hong Kong on June 1 even if he does face Ulster, but it would make him available to face Western Force four days later. Rowntree was speaking at Tuesday's open practice session at Carton House, the squad's training camp outside Dublin, with some 200 supporters in attendance.
"The lads get a buzz out of it and it puts a bit of pressure on them," England's forward coach said. "We got a couple wrong on our line-out throw because of the added pressure. It was great to see everyone there and the lads were good, staying behind to sign autographs."
Of the 37-man squad selected for the tour to Hong Kong and Australia, 15 are involved in matches this weekend with Leinster, Ulster, Leicester, Northampton and Toulon and have yet to join their counterparts.
Rowntree, himself a two-time Lions tourist, envisages few issues integrating the incoming players when they arrive on Sunday.
"Given the nature and class of those players, I'm sure they'll pick up our systems and the way we do thing very quickly, that's why we're picking them," he said.
"The beauty of how we're doing things is that it's very simple. There's a very simple shape to how we're playing the game, our calling structure is very simple because of the understanding that we have limited time to get things right.
"So the guys coming in will slip straight in and they'll be battle hardened heading into Hong Kong."