Luke Lewis has revealed doctors fear arthritis could be behind a serious knee problem which is blighting his decorated rugby league career.

Lewis returned from the injury in Cronulla’s NRL loss to Melbourne on Saturday, but the recurring injury has become a serious concern for the former Test and Origin star.

Doctor are baffled by the injury, which has at times rendered Lewis, 31, unable to walk and has made him a week-to-week proposition for the winless Sharks.

“For some reason fluid keeps building up behind the knee and the doctors aren’t quite sure why, which is a concern,” Lewis told AAP.

“At times it got so bad I just couldn’t walk on it, but it is progressing now and we hope we are getting on top of it.

“The doctors think it might be rheumatoid arthritis or even gout. We just have to keep doing more tests and get the medication right.

“It’s good to be back playing football, but it is something that can change each week.

“It has been a really, really frustrating time for me.”

Rheumatoid arthritis is an auto immune disease, one in which the immune system targets the body’s healthy tissues causing pain and swelling of the joints including hands, feet, hips and knees.

It is not know what causes rheumatoid arthritis, which affects about one per cent of the world’s population.

Gout is a more common form of arthritis caused by a build-up of a waste product, uric acid, in the bloodstream.

The injury has turned Lewis, who at his best is one of the NRL’s most dangerous ball carriers, into a bit part player for the struggling Sharks.

After another week of meticulous rehabilitation and physiotherapy he will again come off the bench for Cronulla’s must-win match against Gold Coast at Remondis Stadium on Saturday.

“I just want to get out there and concentrate on playing football and not have to worry about my knee and how it is going to be, how it is going to recover,” he said.

“Sometimes you just don’t know what is going to happen.

“But it was a positive to get through the game last week and if we can finally get on top of it, put some games back-to-back that will go some way towards a recovery.

“It will hopefully be something we can manage when we do work out why it is actually happening.

“It’s been a tough start to the season for us and I just want to really get out there and help the boys through it.”