Defender Aaron Mullett is expected to be sidelined until round seven after undergoing surgery on Monday to repair a torn labrum in his left shoulder.

The 22-year-old showed minor shoulder concerns at the end of last season that had not prevented him from playing or training.

The club was confident Mullett would get through 2015 unscathed after having scans reviewed by a specialist.

But director of sports science and training services, Steve Saunders, said Mullett suffered further damage over the last two months and now required surgery to repair the labrum.

"The consensus was that Aaron would be fine to get through another season and we'd review him again at the end of the year," Saunders told NMFC.com.au.

"New symptoms developed, we've reinvestigated these and found new pathology in his shoulder joint."

After a promising start last year, the 184cm backman struggled to solidify a spot in defence, playing just four of the Roos' last nine games before finishing with 17 appearances.