Sydney ruckman Sam Naismith will miss the 2018 season after rupturing his anterior cruciate ligament at training on Wednesday.

Naismith was performing a routine drill before sustaining the injury, which will now sideline him for the entire season.

โ€œObviously this is a disappointing setback for Sam so close to the start of the new season,โ€ Sydney's Head of Football Tom Harley told sydneyswans.com.au.

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

โ€œWhile this is an unfortunate injury for Sam, we know heโ€™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.โ€

Naismith has played 28 games for Sydney in his four-year career with the club, with all but one of those coming in the past two seasons.

The 25-year-old averaged 27.9 hitouts and 8.9 disposals per game in 2017.