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.
Carmarthen Quins are holding an exhibition to commemorate club players who fought in World War I. All welcome to the clubhouse on Friday afternoon to learn more about local history, or share any family anecdotes or artifacts.
Defence coach Shaun Edwards believes Saturday's clash against England - the 'form team in world rugby' - poses a huge challenge for his side but he is confident, with Wales beginning to show the defensive qualities displayed in the last two World Cups,