GREATER Western Sydney has been dealt a cruel with young key forward Jonathon Patton ruled out for the season with a torn anterior cruciate ligament in his knee.

The No.1 pick in the 2011 NAB AFL Draft chased a loose ball towards goal in the second quarter of Saturday's game when he was brought down by St Kilda's Rhys Stanley and his right leg buckled underneath him.

Screaming in pain, Patton was taken from the field and immediately went for a scan, which confirmed the worst.

It is shocking luck for Patton, who missed the bulk of last year recovering from tendinopathy surgery on his other knee and had been showing the benefits of his first AFL pre-season.

"He'll be out for the year," GWS coach Kevin Sheedy said after the game, which the Saints won by 72 points.

"That's football. I've had these sorts of situations before with (former Essendon ruckman) Paul Salmon, he played a great first year and Geoff Raines I think ran across his leg and he missed a year.

"But he came back and became a great player.

"They're the tough rigours of an AFL career … it's a challenge for the young man.

"He's already had an operation on his other leg and this is an injury to his good leg.

"It's just unfortunate, we'll keep him up and keep him around the club, and it creates other opportunities for other players to come in."

Giants general manager of football Graeme Allan said it was very disappointing for Patton and the club.

"Jon worked very hard during the pre-season to get himself fit and had shown what a talented young player he is after missing a large part of last year because of surgery to his left knee," Allan said.

"To suffer another serious knee injury so soon is a cruel blow. Naturally the whole club is right behind him and we will fully support him as he undergoes surgery and through his recovery.

"We have every confidence that he will work just as hard again to get back to full fitness and his absence is an opportunity for other players at the club to step up."

Allan said the club and Patton would determine the best course of action for his rehabilitation.

The injury looms as a significant setback for the Giants, who had been pleased with the fledgling pairing of Patton and fellow spearhead Jeremy Cameron.

It will now be 12 months before they can resume their partnership, a harsh reality due to the time it takes two young key forwards with distinctly different styles to develop.

Sheedy had been thrilled with how Patton and Cameron were growing up front.

"Absolutely. It takes a long time," he said.

"They take a long time to get a pattern of play. (Former Essendon forwards Matthew) Lloyd and (Scott) Lucas were both left-footers, in this situation one was a left footer and one was a right.

"It's like a doubles pairing in tennis, it's good to have one of each.

"It's just very unfortunate for that young man."

Despite the injury, veteran mentor Sheedy was remaining pragmatic and felt it would now give others an opportunity to show what they could do.

Setanta O'hAilpin, Sam Frost and Adam Tomlinson are among the players in contention to slot into the forward line for the rest of the season.

"I've been through it that many times that you sort of know it's going to happen," he said.

"That's the unfortunate part of it and puts the big impetus on that group that was sitting just behind Jon and Jeremy in the forward line structure.

"They'll see their chance."

James Dampney is a reporter for AFL.com.au. Follow him on Twitter: @AFL_JD