His club have confirmed that Lamont will not be able to train fully again until next summer, with the announcement a major blow to Scotland ahead of the RBS 6 Nations Championship campaign.
Lamont, 26, was Scotland's player of the season for 2006-07.
The ligament injury came just 17 minutes into Northampton's National League match.
Northampton have revealed that Lamont must undergo knee reconstruction surgery, given the nature of the damage done.
Lamont has 35 caps for Scotland and was a key man in the squad at the World Cup, where Scotland bowed out in the quarter-finals after defeat to Argentina.
The news of Lamont's injury will force Scotland coach Frank Hadden to rethink his plans for the RBS 6 Nations, given it was a certainty the Saints player would feature.
However, Hadden is not without options, and Nikki Walker could step in at number 14, with Simon Webster also an option. Chris Paterson is another player likely to considered for the role.
Scotland team doctor James Robson has held discussions with his Northampton counterparts after hearing of the severity of the injury to Lamont.
He said: "Sean was referred to a specialist in London and he will have his knee in a brace before undergoing a repair.
"We all wish him well and hope he has a successful rehabilitation."