ST KILDA'S charge for a finals berth has suffered a serious blow, with defender James Gwilt to miss the rest of the season.

The Saints' worst fears have been confirmed, with scans showing Gwilt ruptured an anterior cruciate ligament in his left knee in the clash against West Coast at Etihad Stadium on Saturday night.

Saints coach Ross Lyon confirmed the news on Sunday.

"We've had that confirmed … I spoke to Tim Barbour our doctor, it's definitely an ACL with some sort of lateral meniscus damage,'' Lyon said on 3AW.

Gwilt's injury is a cruel blow to the Saints, who are gathering some serious momentum in the race to the finals, with the win against West Coast the Saints' sixth in eight matches.

Gwilt, who has played 82 matches, has not missed a game for the Saints in 2011 and is averaging 18.9 possessions.