IT COULD be two months before Sydney Swans premiership defender Alex Johnson undergoes his third knee reconstruction after his LARS graft failed on Saturday.

The 22-year-old has endured a rotten run of luck with injury, starting in March last year when he tore his anterior cruciate ligament during the NAB Cup.

However persistent inflammation during his rehabilitation saw the hamstring graft to his knee fail, and in August he underwent a LARS/allograft hybrid operation in a bid to return sooner.

After that graft broke down on the weekend, it is now expected the young backman will have to wait for inflammation and a possible infection in his knee to subside before having a third rebuild - a process that could take two months.  

"I think in normal circumstances, the timeframe required to treat an infection process prior to any subsequent surgery is at least six to eight weeks," leading sports medico Dr Peter Larkins told AFL.com.au.

Johnson will have a scan on Wednesday and arthroscopic surgery next week to treat the inflammation and remove the broken pieces of the LARS graft.

Dr Larkins said the graft's failure was related to the on-going inflammation Johnson suffered last year, which coach John Longmire acknowledged had been a problem.

"Since his first operation, he's had swelling in the knee since then," Longmire said.

"In fact he had repeated swelling and it wasn't until we went back we found out the graft hadn't taken.

"He's had that (swelling) from day one and we thought we'd got on top of it.

"We'll work out in the next few days what we're going to do.”

Dr Larkins said because the swelling was related to the initial graft failure, it was important the problem was rectified before another reconstructive operation.

"The dilemma now is Alex needs an extended period of time to get the inflammation and infection settled down in his knee," he said.

"He won't undergo further reconstruction surgery until the club is happy his knee is ready."

Longmire said it was easy to feel for the defender after he so diligently applied himself to rehab last year.

He said Johnson had looked "bloody good" for the 10 minutes he lasted in the Swans' reserves practice match on Saturday and had been training strongly.

"Outwardly he's pretty resilient and putting on a brave face and putting on a really strong outward approach to how he's going to move on to the next part of the operation and then the rehab," Longmire said.

"Inwardly, as you would expect, he's devastated and disappointed and it'll take a few days for him to get over that feeling before he gets into the next phase.

Larkins said the swelling and possible infection throughout Johnson’s rehab had increased the chances of the graft breaking.

"It would affect any outcome of surgery regardless of the type of procedure he had."

Larkins said it would be a surprise if the Swans decided to install a second LARS graft in Johnson's knee and expected they would take the traditional approach this time.

"With a failed LARS, to do a LARS again … it would be most unlikely. I would be really surprised if they considered it under the circumstances," he said.

"It's more likely he'll follow the same route Daniel Menzel has followed at Geelong, where he had a two-stage operation.

Menzel's fourth reconstruction took place over three months last year, with the first operation removing the elements of his previous graft and second rebuilding the ligament.  

Dr Larkins also said it was unlikely Johnson would play again in 2014.