Alex Johnson's fairytale return to AFL football could be over after just two games after the Sydney defender suffered a suspected serious knee injury late in the first term of the Swans' clash with Melbourne at the MCG on Sunday.

Johnson's "good" right knee appeared to buckle when he went to change direction as he chased Demons spearhead Jesse Hogan near the boundary line.

The Swan was assisted from the ground by two trainers and did not appear to be able to put weight on his leg.

Sydney football manager Tom Harley told Channel Seven at the first break the club suspected Johnson had suffered yet another ruptured anterior cruciate ligament

"It's the knee he hasn't had any trouble with at this stage. It looks like an ACL (injury) which is disappointing obviously for everyone," Harley said.

"He's certainly a much-admired character so it's disappointing for him."

Swans coach John Longmire said Johnson's injury meant there were mixed emotions in the rooms after his team's brave nine-point win.

"It's a strange feeling for everyone to be honest. You have just had a great win and you really reach down into the depths of your character for that win and we were thrilled with it and you come off the ground and Alex has got a suspected ACL on his good side so everyone digesting that, including Alex," Longmire said.