DEFIANT Greater Western Sydney defender Nick Haynes denies the Giants' injury crisis has ended their finals chances and says the club is pushing forward, beginning with Saturday's crunch game against North Melbourne.

Giants co-captain Phil Davis (ankle) and fellow key defender Joel Patfull (lung/ribs) will miss 6-8 weeks apiece after being injured in last week's disastrous 42-point loss to Collingwood at the MCG.

Ruckman Shane Mumford (ankle) also went down and will miss the rest of 2015, while midfielder Stephen Coniglio (shoulder) is also in the rehab group for at least a fortnight.

Mumford's loss is a massive blow for GWS, and while the absence of their two key men down back has many writing off the Giants' chances of a maiden finals berth, Haynes has welcomed the doubters.

"We like being the underdogs, we love a challenge. Obviously Shane Mumford, Phil Davis, Joel Patfull and Stephen Coniglio are very good, experienced players, but we're looking forward to the guys coming in," Haynes told AFL.com.au.

"We're expecting them to play some good footy in the ones and we're looking forward to the challenge, nothing has changed."

Haynes and Aidan Corr, who will take over from co-captain Davis and dual Brisbane Lions club champion Patfull, have played 59 senior games between them.

The pair will be joined in defence by debutant Caleb Marchbank, the youngest player on the club's list, when the Giants take on the Kangaroos this week.

On paper, the visitors' three big forwards Drew Petrie, Ben Brown and Jarrad Waite look to be an ominous trio for the young GWS backs to counter, but Haynes is confident the young Giants can do the job.

"It's nothing new with myself and Aidan (Corr); he's used to playing on those big talls and I play on them every now and then and try and learn as much as I can," Haynes said.

"All we can do is go out there and try our best. We're going to need a lot of help, not just from our backs but the midfielders as well, and hopefully we can put a team defensive effort together."

Marchbank, 18, was drafted at pick six last year, and has impressed Haynes with the way he has attacked his debut season in the AFL, both at training and in the NEAFL.

"He looks like a player that doesn't just care about his own man, he worries about the whole defence, and they're the type of players we want back there," Haynes said.

"For a first-year player his level of competitiveness is pretty amazing. He's a very selfless player and has a lot of courage, so I reckon he'll be a very good player."

With Davis and Patfull in the stands, 202-game veteran Heath Shaw's experience will be even more vital for GWS - and Haynes, Corr and Marchbank in particular -over the coming months.

Haynes said the 2010 premiership Magpie had been outstanding for the Giants' defenders since he headed north at the end of the 2013 season.

"Leadership is massive, especially in the backline, and his voice and direction has been good ever since he came to the club, and it's really helped us a lot," Haynes said.

"It helps us learn, and we're going to need his direction, but we've all got to step up and take a leadership role in the next few months."