Firrito received a knock from teammate Ben Warren's knee during last Friday's training session, and rolled the ankle in the same incident.
Club doctor Con Mitropoulos said Firrito's ankle had settled by Saturday but soreness had returned after a five-minute trial on Monday.
"He felt pretty good and ran around a little bit, but when the twisting and turning started he could still feel it in that ankle joint," he said.
"The ankle hasn't swollen up at all but I'm sure he's got a good bruise on the bone. More than likely, because he twisted the ankle at the same time, he's just given a little tweak to the internal ligament - the one that holds the tibia and fibula together ... just a little scare.
"Generally with these, I like to MRI (scan) them to plan when they (the player) should return."
Mitropoulos said Firrito had been able to take part in most other exercises and should only need to rest the ankle for a week from this Friday before he resumed running on it.
He added that the 26-year-old was in no danger of missing intra-club matches or North's NAB Cup opener against Geelong on February 21.
"He has not missed a single thing up until this point, so it's perhaps not a bad idea for him to unload himself just a little bit," Mitropoulos said.
"Bone bruises can take a week or two to settle down but I don't anticipate he'll miss much at all."
Mitropoulos confirmed first-round draftee Ben Cunnington had been able to walk without a moon boot for stages of the camp as he continued his recovery from a foot stress reaction.
Cunnington, who was diagnosed with the injury on January 6, should lose the boot by the end of the week but will still be a fortnight from running.
"I'm keeping him in it (the boot) for the full 14 days but I can't feel any sore bits at the minute," Mitropoulos said.
"He's a bit of a thoroughbred. He just can't wait to get going. Sometimes I pull the reins back on these young kids and keep them a few extra days just to be conservative.
"There's no great urgency at the moment."
The 18-year-old is expected to rejoin his teammates for full training around the middle of next month.
North's camp ends on Wednesday with what will be its hardest session of the seven days.
