"It doesn't look great. I'm not too sure at this stage, it's similar to the first one," Johnson told Channel Seven after the match.
Johnson's injury overshadowed the Swans' win with players emotional in the dressing rooms.
Swans coach John Longmire told his charges they needed to pick themselves up from the pain of seeing their teammate suffer another serious injury.
"He said it was a great effort. We've got a choice now. We can walk out of here and feel sorry for ourselves – AJ wouldn't want that – or we can dig in and make the most of the rest of the season because there's a lot to play for and I know Alex, that's what he'd want," Swans captain Josh Kennedy said on 3AW. "Resilience was the theme in the second half. He's been the leading example of what that really means. [It] certainly was mentioned, it was good we were able to hold on in the end."
Seemingly down and out a fortnight ago, the Swans have resurrected their season with two heart-stopping wins. They can book a finals berth with a victory in the Sydney derby against a Giants team that lost another three players on the weekend.
The odds were firmly stacked against the Swans when Smith was ruled out of the game shortly after Johnson came to grief. It left them two men down for the best part of three quarters in a match played at a finals-like intensity.
The injuries left them vulnerable in a frantic final quarter when the Demons, who trailed by as much as 28 points, stormed home.
They moved within seven points entering time on when Nathan Jones kicked a goal on the run but couldn't find a way around the Swans' extra man in defence Isaac Heeney.
Heeney took several key marks deep in defence, including a screamer on the shoulders of Melbourne key forward Jesse Hogan. It was fitting the blond star had the ball in his hands when the final siren sounded.
While the win over Collingwood was inspired by a virtuoso performance by Lance Franklin, this triumph was the product of their trademark grit and determination.
Captain Josh Kennedy was inspiring with 17 possessions in the second half while youngsters like George Hewett and Nic Newman showed poise beyond their years.
Dan Hannebery played arguably the best game of his season in his first game back from injury. Franklin was again influential, kicking two goals five but was not dominant.
Melbourne, however, will be ruing their wastefulness in front of goal. They had six more scoring shots and 17 more entries inside 50 but paid a hefty price for their lack of composure around the sticks.