The 25-year-old, a summer signing from Dundee United, came off at half-time in the 2-0 defeat by npower League Two leaders Gillingham and a specialist confirmed he damaged his anterior cruciate ligament.
The Scot will undergo an operation on Friday and manager John Ward has said he is looking at bringing in another defender.
"This comes as another massive blow to the club, and we all wish Garry a very speedy recovery," an official club statement said.
- Sports & Recreation
- Dundee United