Sam Naismith misses season after rupturing ACL

Sydney's AFL flag hopes have taken a big hit, with No.1 ruckman Sam Naismith sustaining a season-ending knee injury.

Naismith, 25, ruptured an anterior cruciate ligament during a training drill on Wednesday.

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

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

The Swans have gone from a glut of front-line ruckmen to a dearth of options in less than two years.

Toby Nankervis was traded to Richmond at the end of the 2016 campaign and is now a premiership player with the Tigers, while Kurt Tippett retired in January due to a serious lingering ankle injury.

With Naismith gone for the year, Callum Sinclair is the only available experienced ruckman on the Swans' list.

Sinclair hurt an ankle in Sydney's pre-season match against Brisbane Lions but assistant coach Steve Johnson said on Wednesday the 28-year-old was in no doubt for the home and away season opener against West Coast on March 25.

The 205cm Naismith enjoyed his best season to date in 2017, playing 15 games for a career tally of 28.

"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," Harley said.