The 35-year-old's absence could force Seagulls boss Micky Adams to move into the loan market to sign a temporary replacement.
Adams told the Argus: "We were worried about his cruciate ligament because he's had cruciate knee damage before.
"The surgeon has repaired the cartilage. There's significant bleeding and we won't know the extent of the damage until that's settled down, but the surgeon seems to think, if everything goes well, it will be four to six weeks.
"We're disappointed with that obviously, as is Nicky. I'm looking for a 90-day loan if I can get one but a lot of clubs are talking about 28 days.
"I'm slightly loathe to get in another player on a month's loan and then send them back because I don't think that benefits the squad."