Naismith was performing a routine drill before sustaining the injury, which will now sideline him for the entire season.
“Obviously this is a disappointing setback for Sam so close to the start of the new season,” Sydney’s Head of Football Tom Harley told sydneyswans.com.au.
“Sam was involved in a fairly routine training drill when his knee innocuously 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.”
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.