The Wellington Phoenix will be without their captain for the remainder of the A-League season.
Scans have confirmed Alex Rufer has ruptured the anterior cruciate ligament in his left knee.
He will have surgery in four to six weeks and faces up to 12 months of rehabilitation.
Rufer suffered the injury in the first half of Saturday's match against Brisbane Roar when he attempted to make a tackle on Roar midfielder Rahmat Akbari.
It is a bitter blow for the 25-year-old, who has been in good form at the heart of the Phoenix midfield.
Rufer has started all 14 of Wellington's A-League matches this season, playing 90 minutes in all but one of their games during their recent seven-match unbeaten run.
He also played a major role in the Nix's run to the semi-finals of the FFA Cup, deputising at centre back in their dramatic quarter-final win over Melbourne City.
Phoenix head coach Ufuk Talay says it's a major disappointment for Rufer and the team.
"We really feel for Alex as he has been playing some of his best football in recent weeks," Talay said.
"It's also a major setback for the team to lose their captain midway through the season.
"Alex has grown into a fantastic leader and will be missed, but it's an opportunity for other players to step up in his absence."
The Wellington Phoenix will have a new captain for Friday night's match against the Newcastle Jets with vice-captain Oli Sail in Qatar with the All Whites.