The defender was playing in just his second AFL match since the 2012 Grand Final, where he sustained the injury in the opening quarter.
Johnson, 26, has previously had five separate knee injuries in the past six years, with scans today confirming Johnson’s worst fears.
Sydney Swans Head of Football Tom Harley said he and the club as a whole is disappointed for Johnson.
“Alex has worked so hard over the past six years to get back to playing football and for this to happen in his second game back is just shattering for him and for us,” Harley said on the club website.
“Alex has shown unbelievable resilience and determination over his career and will continue to be an inspiration to his teammates and the wider AFL community.
“As we saw after the injury on Sunday, Alex is the ultimate clubman and will be doing all he can to support his teammates from the sidelines for the remainder of the season.”
Meanwhile, Nick Smith is set to miss the next six weeks due to a hamstring tendon injury which he too sustained in the opening quarter of the clash with the Demons.
Harley said it’s a terrible setback for the defender.
“Nick’s got a hamstring tendon injury, at this early stage it looks like six weeks for Nick which obviously makes the season challenging,” Harley said.
“He had a scan yesterday, we’ll gather all of the facts as to the next steps with Nick.
“Clearly it was disappointing he wasn’t able to play any further part in the game on Sunday and it looks like he’ll be having an extended period on the sidelines also.”