It is not the first time O'Brien has struggled with injuries, and with that in mind Leinster are not going to rush his return, with the flanker expected to be on the sidelines for at least another couple of months.
"I think the medical staff see [O'Brien] as someone they need to be very careful with," said assistant coach John Fogarty.
"With everything that has happened in the past, they are bearing that in mind as they bring him back. Once he hits all his markers - and trust me it is very scientific. Three days a week we get medical reports.
"Sean's is a 20-week recovery from the surgery.
"I believe we are about six to eight weeks into it. That's what we're hearing at the moment. But it depends on whether he hits markers, if he doesn't hit markers it slows things down.
"Believe me it is frustrating for us [too].
"He is one of the guys who has found his voice and is really leading even though he is not on the pitch."
New Leinster signing Robbie Henshaw will also be out for the start of the Guinness PRO12 season, however Cian Healy is now back in training and could be fit sooner rather than later.
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