GREATER Western Sydney defender Zac Williams will have surgery on his ruptured Achilles on Tuesday and is likely to miss most of the 2018 season.

Williams went down at training on Friday morning and had scans to confirm the seriousness of his injury on Monday.

ANALYSIS: How does GWS cover Zac Williams' loss?

An exact date for the dashing backman's return is unlikely to be known for some time, but the Giants are preparing to be without the 23-year-old for most of the home and away season at best.

Williams, who has become a permanent member of the GWS defence over the past two seasons, played 45 of a possible 49 games in that time and established himself as one of the game's best rebounding half-backs.

The first Giants academy graduate to play senior footy when he came off the rookie list to make his debut against Gold Coast in round five of 2013, Williams averaged a career-high 22.2 possessions per game in 2017 and finished sixth in the best and fairest.

The loss of Williams is a cruel blow for the Giants, who traded his fellow attacking defender Nathan Wilson to Fremantle during last year's player exchange period.

Adam Kennedy and Matt Buntine could have been reliable replacements, but both are recovering from knee reconstructions and are unlikely to be ready to return to senior footy for at least the early part of the season.

The injury will place a huge workload on veteran running defender Heath Shaw.