The 28-year-old striker, who has been blighted by injuries in recent seasons, returned to the first-team fold in December, but has struggled for fitness since the turn of the year.
"He's done his cartilage and he's had an operation. It's very unfortunate for him. I don't know exactly how serious it is at the moment, but he needed it done," McDermott told reporters at a press conference.
The South African, who joined the club in 2009, initially suffered an anterior cruciate ligament injury in July 2011.