Sydney ruckman Sam Naismith's wretched knee woes appear to have deepened with the Swans fearing he has ruptured his ACL for a second time. 

Naismith, playing just his 30th AFL match, finished Thursday night's loss to the Western Bulldogs on the bench with a right knee injury sustained in the final term. 

The 27-year-old will head for scans on Friday with the Swans bracing for confirmation of a second ruptured ACL. 

04:59

Longmire post match - Rd 4, 2020

John Longmire reveals that returning ruckman Sam Naismith has been dealt a cruel blow after injuring his knee in the final term.

Naismith underwent his first knee reconstruction in the pre-season of 2018 and sat out the rest of that season and then the entire 2019 campaign with further complications. 

"We'll confirm it with scans, but it looks like he's done his knee," Swans coach John Longmire said post-match. 

"It was in the last quarter at some stage, I'm not exactly sure, I only spoke to him briefly. 

"He's missed two years of footy and comes back and plays round one and plays really well. 

"He lost his sister through that (shutdown) break and then he comes back and does his knee. It's a terrible run for him." 

The Swans' ruck depth consists of Callum Sinclair (199cm) and Hayden McLean (197cm), both of whom played in the loss to the Dogs, plus rookie Michael Knoll (205cm) and Joel Amartey (197cm).