North football director Geoff Walsh told NMFC.com.au on Monday that scans had revealed Macmillan had suffered a non-displaced fracture of the fibula, but would not require surgery.
"Probably in that sort of situation it's the best possible result, (it will be) maybe four to six weeks before he's back playing," Walsh said.
"No one ever likes to suffer an injury and particularly a break, but in terms of that (it's) not a bad result."
Macmillan, 22, was injured midway through the third term of North's seven-point win over the Power, landing awkwardly after spoiling Port midfielder Hamish Hartlett.
His injury breaks a run of 49 consecutive games that began in round 24, 2011.
Walsh said Macmillan would use a moon boot and crutches for about a week before he started putting weight on his right leg.
"It will be treated symptomatically. As he feels comfortable, he'll be able to take more and more weight on it," Walsh said.
Meanwhile, Walsh was hopeful North vice-captain Jack Ziebell would return this Sunday against the Sydney Swans after being a late withdrawal from the Port game with a right shoulder injury.
WATCH: Behind-the-scenes vision of Ziebell's fitness test
"I don't think Jack missed by all that much (from playing against Port)," Walsh said.
"We'd expect that he becomes more comfortable as the next 48 hours roll on and more than likely gets out there and trains and presents himself as available for selection against the Swans this week.
"Rather than a fitness test, I think we'd just like to see him present for training towards the middle end of the week."
Ziebell himself was quietly optimistic he would recover in time to take on the Swans.
"I’m feeling pretty good and hopefully a chance this week. Hopefully (I'll) train this week," Ziebell told Channel Nine's Footy Classified as he headed for scans on Monday.
"(I’m) pretty close, about 80 per cent (at the moment) with some improvement to do and getting better every day.
"Obviously (I) have to be feeling 100 per cent fit to play."
Walsh said key forward Robbie Tarrant (tibia) remained on track for a round seven return, having increased the volume of his running last week.
Taylor Garner, the Roos' first pick in the 2012 NAB AFL Draft, suffered a left hamstring tendon injury in North's NAB Challenge loss to Hawthorn, but Walsh said he was on track to play in round six or seven.
Walsh said 2013 draftee Ben Brown (knee) was two weeks away from resuming full training, with round seven his likely return date.
However, the news is better for tall defender Cameron Delaney (toe/foot), who will be available for VFL selection if he gets through training this week.
Twitter: @AFL_Nick
