Will require a period of rehabilitation between 12 to 16 weeks
Wales and British and Irish Lions captain Sam Warburton will be sidelined for up to four months because of his left hamstring injury.
Warburton was hurt late in the second Lions Test against Australia on June 29 in Melbourne, and had a scan in London on Thursday.
Cardiff Blues said on Friday the flank had a grade three tear.
"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," Cardiff Blues head of medical John Miles said in a statement.
That means Warburton might not be ready to play until October.
Wales' first home Test in November is against South Africa on November 9.