"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."
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.