The Australian-born second-rower was stretchered off in the first half of Wales' scratchy 11-10 win over Fiji on Friday night. His hamstring was torn away from the bone in a sickening collision and he may not play again this season.
Wales coach Mike Ruddock confirmed: "The injury needs an operation and Brent is out of action for at least four months, possibly six.
"It's awful news for Brent and his family, as well as the Ospreys and Welsh rugby, who have lost a quality player.
"On the injury front, we are getting a lot more headaches than we would like."
Cockbain, who turns 31 on Tuesday, joins fellow British Lions Gethin Jenkins, Ryan Jones, Dwayne Peel, Tom Shanklin and Gavin Henson on Wales' ever-expanding injury list. As well, three other Wales players - Dafydd Jones, Kevin Morgan and Chris Horsman - limped off during the Fiji match with ankle injuries. Jones and Morgan are unlikely to play against South Africa next Saturday.
The Welsh Rugby Union has awarded a unique 'President's Cap' to a specific group of players who represented their country between 1945 and 1979, but missed out on international recognition at the time.
Prabhat Mathema, the WRU's National Medical Manager explains the four part Pitchside Suspected Concussion Assessment process the IRB has devised that is being implemented throughout the Dove Men Series and in the LV= Cup this season, which includes the provision for a five minute temporary substitution to allow an assessment to take place.