GREATER Western Sydney has been dealt a massive pre-season blow with damaging half-back Zac Williams suffering a ruptured Achilles at training on Friday morning.

While the full extent of the injury is unclear, the Giants expect the 23-year-old will be sidelined for an extended period of the 2018 season.

Williams, one of GWS' best ball users, will see a specialist on Monday to determine if surgery is required.

"This is a really unfortunate injury for Zac who had been training really well throughout the pre-season so far," head of athletic performance David Joyce said.

"Zac will meet with a specialist early next week to determine the extent of the injury which will allow us to then map out the best possible management pathway." 

Williams' injury compounds the loss of fellow running defender Nathan Wilson, who was traded to Fremantle at the end of last season, leaving the Giants with gaping holes to fill across their half-back line. 

Possible replacements include veterans Brett Deledio and Ryan Griffen, or untried 2016 draftee Isaac Cumming, who won the club's NEAFL development award last season. 

Deledio (calf/Achilles) and Griffen (ankle) are currently on modified programs after struggling through last season, but both are on track to resume full training in the next month. 

Williams joins several other AFL players to have ruptured Achilles in recent years, including Hawthorn captain Jarryd Roughead, now-retired North Melbourne midfielder Andrew Swallow and young St Kilda tall Hugh Goddard.

Roughead's 2011 campaign was ended when he suffered his injury in round 12. The star Hawk returned to action in round two the following year. 

Swallow went down in round 18, 2013 and didn't make his comeback at AFL level until round nine the next season.

Goddard tore his Achilles tendon in June 2016 and was sidelined for 10 months after undergoing surgery.