The Dubliners are waiting until later today at the earliest to give a definitive prognosis of any damage but it is likely that the 31-year-old will miss at least two weeks of action.
"We don't know for sure but I don't think it's going to be as bad as we first feared," said Leinster head coach Joe Schmidt.
"When I first saw him come off I thought it was four to six weeks straight away.
"He might only be out for two or three weeks, maybe four weeks, although this is still only an estimate at the moment. We'll do some more tests and get a clearer picture."
If Schmidt's more optimistic prediction proves to be realistic, then O'Driscoll will have to sit out next weekend's trip to Wembley for the second round of continental action, as well as what should be a mouth-watering all-Irish clash with Eric Elwood's high flyers at The Sportsground on October 23.
He would also be unlikely to feature in the following week's fixture against Edinburgh at the RDS, the final league match before Ireland begin their autumn international series against South Africa on November 6.
Worryingly for Ireland boss Declan Kidney, if the injury proves to be as bad as Schmidt initially feared, O'Driscoll may struggle to play any part in the autumn Tests against the Boks, Samoa, New Zealand and Argentina.
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