GEELONG, AUSTRALIA - JULY 04: Callum Mills of the Swans kicks the ball during the round 16 AFL match between Sydney Swans and West Coast Eagles at GMHBA Stadium on July 04, 2021 in Geelong, Australia. (Photo by Daniel Pockett/AFL Photos/via Getty Images )

Sydney Swans co-captain Callum Mills is in doubt for Round 1 as he battles an ongoing Achilles concern.

The 24-year-old missed the Swan's elimination final loss to cross-town rivals GWS with the same injury and coach John Longmire told it was a problem the Swans were continuing to monitor.

"He's worked so much to get that Achilles right," Longmire said.

"Often with these injuries you take two steps forward and one step back. That's happened a little bit over the pre-season with him."

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Mills was appointed as one of three co-captains earlier this month, replacing star midfielder Josh Kennedy and joining incumbents Dane Rampe and Luke Parker.

But while Round 1 draws nearer, Longmire admitted he was unsure whether Mills would line up against GWS.

"Where that leaves him for Round 1, we're not sure just yet," Longmire said.

"We'll play him when he's right and he's right for the entire season.

"He's been doing some drills but we're really mindful we're not going to push him too hard if it sets him back two to three weeks."

The news of Mills' injury comes days after the club confirmed midfielder James Rowbottom has undergone surgery after breaking a bone in his hand.