Australia and ACT Brumbies openside flanker David Pocock has suffered a major left knee injury and is expected to spend six months on the sidelines, missing the three-test British and Irish Lions series beginning in June.

Pocock, who served as Wallabies captain for part of last season, was injured 13 minutes into the Brumbies' 35-6 win over New South Wales on Saturday in Canberra.

Scans showed he suffered a torn anterior cruciate ligament and he faces a knee reconstruction, putting him out of action for up to six months.

''Not the best news with the scan. I'm going to have to see the surgeon Monday,'' Pocock wrote on Twitter.

Wallabies coach Robbie Deans had already confirmed the 2013 captaincy would be decided between World Cup skipper James Horwill and Pocock, the face of the Lions tour advertising campaign.

Horwill has yet to make his return from injury after being sidelined for 10 months, but is expected back for Queensland Reds in the next couple of weeks.

Pocock, who missed Australia's November tour to Europe with a knee injury, was replaced Saturday by 110-test veteran George Smith, returning to Australian rugby from Japan.

Brumbies captain Ben Mowen was disappointed by the loss of Pocock, who has played 45 tests since his debut in 2008. ''You invest a lot in your playing group and the friendships that you build and you really do ride the highs and lows with each other,'' said Mowen.

''But in saying that he's an absolute quality guy and if anyone's going to do the recovery right ... he's going to be the guy who's going to do it.''

Brumbies coach Jake White said it was fortunate the Brumbies had signed Smith recently to play most of the Super Rugby tournament.

''It shows you how fate plays out ... it's just fantastic how things have worked out now that George has arrived,'' White said.

''It's not nice on Poey, but at the same time it just shows how important depth is in this competition and when you have depth like George it's even better.''

- AAP