Johnson was taken off on a stretcher early in the opening quarter of the side's 36-point win over Gold Coast at Blacktown Oval on Saturday night, the reigning premiers' first victory in their pre-season campaign.
The 21-year-old was a regular member of the Swans' back six during their successful 2012 campaign, but will take no part in the 2013 season after scans revealed he'd ruptured the anterior cruciate ligament in his left knee.
Johnson will now take some time away from the club to decide whether he will have a traditional knee reconstruction or undergo the LARS procedure.
"It's been a pretty hectic 24-to-48 hours since it's happened and I've given it a bit of thought," Johnson said on Monday.
"I think just getting down to Melbourne and getting out of Sydney and out of the club and spending time with mum and dad and family and friends will get my mind off it and will give me a good chance to make the right decision for myself and the footy club."
Longmire on Saturday was philosophical about the major disruption to his season-proper planning.
"He (Johnson) is a very good player ... he's a third tall defender who can also play on the smalls. But unfortunately these things happen," he said.
"It's just football. It happens every year and it's an unfortunate thing for him and us - we'll certainly miss him if he's out. But you can't do much about it."
Rookie-listed Dane Rampe, mature-age draftee Dean Towers and Tony Armstrong are the leading contenders to take Johnson's place in the Swans' side for their season-opening clash with Greater Western Sydney on March 30.