The striker, who was the club's top scorer last season, suffered medial ligament and cartilage damage in a pre-season friendly and he went under the knife after the problem was worse than first feared.
Boss Steve Davis told the club's website: "Having the operation was the right thing to do because we needed the surgeon to have a look at what the trauma has caused.
"It doesn't matter at whatever level you play at, it is always a huge blow to lose your leading goalscorer. It would be the same if Robin van Persie was out injured at United.
"We are all gutted for Mathias and it will be difficult to fill in what we gave us with the power, strength and the goals we got for us."
- Sports & Recreation