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, a regular member of the Swans' back six during their successful 2012 campaign, looked to be in good spirits after emerging from the rooms at halftime.
He limped up the stands, clambered over seats and shared a laugh with teammates.
But Longmire is expecting scans will reveal anterior cruciate ligament (ACL) damage.
"We'll confirm that with a scan but (team doctor) Nathan Gibbs is pretty sure it's a knee reconstruction. It's terrible news for him," Longmire said after his side eased to a 11.14 (80) to 6.8 (44) win over the Suns.
"We have to wait for the scan to make sure it's gone, but Gibbsy usually knows pretty well.
"There's never a good time to do a knee, but when you've been able to do most of the (pre-season) training and you're on the eve of the season - that's certainly not the best time to do it.
"He's just turned 21 - so he's a young bloke and it wouldn't have hit him yet.
"I imagine it might be a few weeks until it finally hits you, when you realise you've done a knee."
Longmire 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.
Rebounding backmen Rhyce Shaw and Nick Malceski both returned from niggling calf injuries on Saturday against the Suns, but Marty Mattner (hip) and Nick Smith (knee) are still in doubt for round one.
Shane Mumford (ankle) and dual Brownlow Medallist Adam Goodes, playing for the first time since suffering a serious knee injury in last year's grand final, survived their first 2013 hit-outs.
Goodes moved well and kicked two goals, playing two and a half quarters before he was substituted for Mumford.
The Swans seized control of the contest in the second term with six consecutive goals.
Josh Kennedy (22 contested possessions, 12 clearances) and Jarrad McVeigh (35 disposals, eight clearances) were dominant in the engine room.