Sydney have confirmed that co-captain Callum Mills is facing up to six weeks on the sidelines due to a calf injury.

Mills suffered the injury in Saturday's loss to Fremantle, subbed out in the opening half, with scans now confirming the Swan has suffered a "moderate grade calf strain".

The 26-year-old, who has played all nine games for Sydney so far this season, is now expected to miss four to six weeks.

The Swans also confirmed that Logan McDonald is facing time on the sidelines after suffering a medial ankle sprain.

The young forward suffered the injury in the second term of the Round 9 clash, now expected to be sidelined for around eight weeks.

Sydney confirmed the news on Monday, with executive general manager of football Charlie Gardiner confirming both will be facing extended time on the sidelines.

"Callum is obviously on of our on-field generals and Logan has been building into some good form so we are disappointed to lose them for what looks like an extended period," Gardiner said.

"We know they will both throw themselves into their rehabilitation and do everything they can to get back as quickly as possible."