No official diagnosis has yet been given on the extent of the injury but the 31-year-old could face a race in time to be fit for the November Tests against South Africa, Samoa, New Zealand and Argentina.
"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, O'Driscoll will have to sit out next weekend's Heineken Cup trip to Wembley to face Saracens, as well as the Magners League clash with Connacht at The Sportsground on October 23.
He would also be unlikely to feature in the following week's Celtic 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 be unavailable for the first, or even all four, of the November Tests.