Lions captain Warren Whiteley will only return to Super Rugby action in April.
The Lions confirmed that Whiteley suffered a "grade two PCL [posterior cruciate ligament] injury and is expected to be out of action for up to four weeks".
The 30-year-old picked up the injury in his side's 35-38 loss to the Blues at Ellis Park on Saturday.
The Lions have won three of their four games in the competition so far and they currently sit at the top of the South African conference.
The Lions face the Sunwolves in Johannesburg in Round Five before they travel to Argentina to take on the Jaguares in Round Six.