THE LOSS of gun defender Zac Williams is a devastating blow for Greater Western Sydney.

Underrated by most of the footy world, Williams will have surgery on his ruptured Achilles on Tuesday and is extremely unlikely to play any part in this season.

Williams might not have the reputation of veteran teammate Heath Shaw, or last year's All Australian half backs Rory Laird and Sam Docherty, but make no mistake, the 23-year-old is one of the Giants' most important players.

He's at home locking down on a dangerous small forward, but it's his ability to impact games in multiple roles that is one of his biggest assets.

With GWS battling a crippling injury toll midway through last year, coach Leon Cameron swung Williams into the midfield. He responded with 28 possessions and six clearances, the last of which helped set up the sealer for Toby Greene in a massive round 10 win over West Coast in Perth. 

One week later with Steve Johnson and Devon Smith missing, Williams was sent forward and bagged three important goals, one of them late in the final term, to go with seven tackles and 25 touches, as the Giants held off a plucky Essendon at Spotless Stadium. 

The Giants eventually got some important players back later in the season, and Williams was back in his usual position in defence, torching the Western Bulldogs in round 21 with 10 rebound 50s and 27 disposals.

Averaging 22.2 possessions per game last year, Williams finished sixth in the best and fairest and polled a career-high six Brownlow medal votes.

He ended the season ranked 24th overall in the Schick AFL Player Ratings, behind only star forward Toby Greene (14th) at the Giants, and Hawthorn gun Shaun Burgoyne (19th) in the category of small/medium defenders.

The absence of Williams is heightened given Nathan Wilson is now with Fremantle after seeking a trade back to Perth at the end of last season, and Shaw is nearing the end of his career. 

Ahead of his 250th game last year, Shaw told AFL.com.au that he saw Williams and Wilson as the future of the GWS half-back line and he was already preparing for the youngsters to take over his role. 

"Those two are tracking a year or two ahead of where I was at that age, over the last 12 months they're getting more confident at knowing their game and directing those around them," he said at the time. 

The Giants have a decent list of replacements, with veterans Brett Deledio and Ryan Griffen, utility Sam Reid, and youngsters Isaac Cumming and Harry Perryman all sure to be looked at, but his speed and willingness to take the game on will be missed.

Reliable pair Adam Kennedy and Matt Buntine could eventually slot back into the Giants best 22, but both are on the comeback from knee reconstructions and won't be ready to play senior footy for at least the early part of the season. 

GWS coach Leon Cameron will likely use a scheduled scratch match against the Swans in February, and the upcoming JLT Community Series, to find the right man to be part of his new look backline. 

But Williams' long-term absence is an unwanted headache after his side suffered so many serious injury problems last year.