SYDNEY'S ruck stocks have been dealt a massive blow after Sam Naismith suffered a serious knee injury at training on Wednesday.

Naismith, who was a late withdrawal from last week's JLT Community Series win over Brisbane with back soreness, went down shortly after the Swans' match simulation began and was helped off the track by Swans medical staff in clear pain.

Scans have confirmed the 25-year-old will require a full knee reconstruction and will miss the 2018 season.

Naismith dropped to the ground and clutched his right knee immediately after what seemed to be an innocuous contest and struggled to put any weight on his right knee, before being assessed by club doctor Tom Cross.

The big man spent several minutes on a rub down table being looked over by Cross and left the ground on crutches with the knee heavily iced.

The news is shocking luck for Naismith, who has managed just 28 games in his five seasons because of chronic groin problems, and knee injuries.

"Obviously this is a disappointing setback for Sam so close to the start of the new season," Sydney's head of football Tom Harley said.

"Sam was involved in a fairly routine training drill when his knee twisted, causing a rupture of his anterior cruciate ligament.

"While this is an unfortunate injury for Sam, we know he’s an absolute professional when it comes to his preparation and recovery and he will no doubt do everything possible to make sure he recovers from this injury."

The loss of Naismith, coupled with the sudden retirement of Kurt Tippett in late January, leaves the Swans with Callum Sinclair and Darcy Cameron as their only ruck options.

Making matters worse, Sinclair is unlikely to play against the Giants in Friday's JLT Community Series clash at Blacktown after he suffered an ankle injury against Brisbane.