ALEX Johnson requires a third knee reconstruction in just over 12 months as the luckless Sydney Swan added a devastating injury to Saturday's shock loss to Greater Western Sydney.

Playing in the reserves in his first official game since starring in the 2012 Grand Final, Johnson showed some positive signs at Spotless Stadium before disaster struck once again just 10 minutes into the game.

First injuring his knee in last year's NAB Cup campaign, Johnson's initial traditional reconstruction failed, leading him to undergo the LARS procedure in August.

But that has also failed, putting the 22-year-old in a desperately sad club of players to repeatedly suffer the dreaded injury which includes Geelong's Daniel Menzel (four times) and Fremantle's Anthony Morabito (three).

"'AJ' requires another knee reconstruction. He lasted 10 minutes," a glum Swans coach John Longmire said after the match.

"He took two intercept marks in the first 10 minutes, at one stage he went down the ground and took about five bounces and looked fantastic.

"Then at the 10-minute mark he went up for what looked like a simple third-man contest in the back half and landed awkwardly.

"'Gibbsy' (Nathan Gibbs) our doctor said he requires another one.

"It's really sad for him. He had two last year, the first didn't take, and clearly this one didn't last much at all."

Co-captain Kieren Jack was a late withdrawal for the Swans, having woken up on Saturday morning with a stiff back.

Lewis Roberts-Thomson then picked up what was described as "a slight groin strain" late in the third term and was unable to continue.

But the worst news of the day was clearly Johnson, a likeable youngster who has already experienced the highest of highs and some devastating lows in his young career.

Johnson was one of the Swans' key players in their premiership, playing every game in 2012 and being an important contributor in the Grand Final against the Hawks in only his second AFL season.

That came after 20 games in his debut year after being drafted from the Oakleigh Chargers in the TAC Cup.

In January, Johnson revealed to the dark times he had gone through after discovering his first surgery was not a success.

"You make little goals if you've got such a long injury and I knew I had nine months ahead of me to get back to full training, and running was a major step. I thought 'here we go'. Then to have that setback was shattering," Johnson said.

"I assessed my options and if I had another traditional I'd miss another half of this year and I wasn’t really prepared to do that.

"I was pretty close to getting LARS the first time, so I decided to go that way and it's been perfect ever since. I walked out of hospital a few days later and the difference is unbelievable."