SYDNEY ruckman Sam Naismith's long wait to return to the field is likely to be delayed again after undergoing a knee arthroscopy on Wednesday.

Swans coach John Longmire hoped as recently as early February that his No.1 big man would be back playing in the first six weeks of the season, but that now seems unlikely.

AFL.com.au understands Naismith, who last played in Sydney's losing semi-final in 2017, developed soreness in his right knee as he ramped up his training intensity.

The 26-year-old ruptured the anterior cruciate ligament in the same knee in a training incident in March last year that subsequently sidelined him for all of last season. 

Naismith will be temporarily sidelined from training and his return date is unclear.

The news comes as Callum Sinclair, who competently filled in for Naismith as the Swans' chief ruckman last year, moves closer to securing a fresh multi-year deal.

Sinclair is expected to re-sign for either two or three years in the coming weeks, and will be crucial this season if Sydney is to buck popular predictions and qualify for the finals.

The 200cm ruck-forward enjoyed his best season in 2018, averaging 14.6 disposals and 29 hit-outs, on top of kicking 12 goals in 23 matches.

Sinclair, 29, joined the Swans on a four-year contract via the 2015 NAB AFL Trade Period in a straight swap for Lewis Jetta, who went to West Coast. 

He will share the ruck duties with defender Aliir Aliir until Naismith returns, with Sydney keen to utilise the former Eagle's forward talents more often next to Lance Franklin and Sam Reid.

Another option for the Swans is to turn to Darcy Cameron, whose only senior appearance to date was in round 18 last year against Gold Coast.