And scans in London have confirmed fears that the Wales flanker will be on the sidelines for between 12-16 weeks from the injury.
"Sam suffered a grade three tear to his left hamstring in the second Lions test against Australia on the 29th of June," said Cardiff Blues Head of Medical John Miles.
"He has had a scan with specialists in London yesterday which has shown the injury is already healing. He therefore does not need surgery but will require a period of rehabilitation between 12 to 16 weeks from the date of the injury.
"His rehabilitation will focus on range of motion, flexibility and specific strengthening of the hamstring in order for him to return to full health."
Warburton could miss the opening rounds of the Blues' campaigns in the RaboDirect PRO12 and Heineken Cup but he is set to be available for Wales' November internationals.