Sydney have suffered a major blow early in their AFL match against Geelong at the SCG, with Kieren Jack suffering a knee injury.

Jack suffered his setback in the first term. Swans football manager Tom Harley confirmed at quarter-time the former captain will play no further part in the contest.

Harley suggested it looks like Jack has damaged his medial ligament. If that diagnosis is correct, it means the veteran is likely to spend at least the next four weeks on the sidelines.