Victory won the FFA Cup and led the league in November but have suffered a form slumped since, with just four wins in 15.

It's worse for Sydney, who are winless in eight and in grave danger of missing the finals altogether.

One man who knows a thing or two about backing up a successful campaign is Australia coach Ange Postecoglou, the only coach to achieve back to back titles in a decade of the A-League.

Postecoglou, who also achieved the feat with South Melbourne in the National Soccer League, said second-season blues for successful teams weren't the exception but the norm.

"It was always going to be harder when you're the champions. There's a target on your back," Postecoglou told AAP.

"For the players, once you've had success you forget how hard it is to attain it. Psychologically there's a chance people get a bit complacent.

"It's really hard to deal with."

Sydney FC may have crossed the Rubicon this season under the weight of injuries, suspensions and an Asian Champions League (ACL) campaign.

Alex Brosque's season-ending hamstring strain means even if Arnold's side scrape enough points to make it to the finals, they face an uphill task challenging higher-ranked teams.

"It looks like he has to rejuvenate that squad," Postecoglou said.

"This year the overseas signings haven't worked.

"It's critical you get them right and that'll be the key to him rebuilding Sydney."

Jacques Faty, Brosque, Chris Naumoff, Ali Abbas, Robert Stambolziev are out of contract and could move on at season's end.

Contracted Visa players Milos Ninkovic, Filip Holosko and Mickael Tavares have all been linked with an early release to their time in the A-League.

Despite that, Postecoglou said the old warhorse Arnold could turn the Sky Blues into championship contenders in 2016-2017.

"Arnie's very experienced, he doesn't need to prove anything to anybody and he's faced these kind of challenges before," he said.

"When you coach for long enough you find there's peaks and troughs."

At Victory, much could depend on whether key foreigners extend their tenure.

Matthiew Delpierre is re-considering retirement at season's end, while Gui Finkler is out of contract and not feeling the love after an ACL squad snub.

Kosta Barbarouses has already flown the coop for Wellington Phoenix and there's speculation Fahid Ben Khalfallah could follow him through the exit door.

Archie Thompson has surely come to the end of a decorated career.

Then there's Carl Valeri, who was struck down with inflammation affecting his brain just a few months into his inaugural season captaining the club.

Mark Milligan - one of just three men to play for both clubs - said it was only natural that Victory would struggle without the man who replaced him as captain, Valeri.

"Carl's very, very important to that Melbourne Victory team and I think it's been obvious how much they've missed him throughout the season," he said.

"It's just been unfortunate. He's one of the best I've played with and a great man so hopefully he gets back playing sooner rather than later."

Milligan backed Muscat to turn Victory's fortunes around; he just wasn't sure if that would happen in time to salvage this campaign.

"I'm sure the club will be fine. They are a big club. There's a lot of pride," he said.

"I have no doubts that whatever situation they're in at the moment they'll turn around and get the best out of it, whether it's before the end of the season or the start of next."

Postecoglou, who preceeded Muscat at Victory, said he was looking forward to seeing how his one-time assistant handled the next few months.

"It's his first rebuild and it will be great for him to go through it," he said.

"Maybe not to Sydney FC's extent, you don't know how many players want to move on of their own volition.

"The one thing Musky has proved is that he's got a good eye for a foreigner. All the foreigners he's brought in have contributed.

"He's shown he's got the right attributes to be a successful manager and if he stays in the game long enough he's going to have to do this a few times.

Revolution or evolution: The rebuilding task facing the A-League's two biggest clubs

WHO COULD LEAVE MELBOURNE VICTORY?

Nick Ansell - Out of contract

Kosta Barbarouses - Gone. Signed by Wellington Phoenix

Gui Finkler - Out of contract and dumped from ACL squad

Giancarlo Gallifuoco - Out of contract and barely used

Jason Geria - Contracted but will attract overseas interest

Matthieu Delpierre - Out of contract and considering retirement

Fahid Ben Khalfallah - Contracted but overlooked for ACL

Archie Thompson - Out of contract and turns 38 this year

Lawrence Thomas - Contracted but not used all season

Carl Valeri - Contracted but serious illness has ended his season

WHO COULD LEAVE SYDNEY FC?

Zac Anderson - Out of contract and barely used this season

Alex Brosque - Out of contract and could retire

Jacques Faty - Out of contract and overlooked for ACL

Milos Ninkovic - Contracted but could be moved on

Chris Naumoff - Out of contract and could attract A-League interest

Filip Holosko - Contracted but overlooked for ACL

Alex Mullen - Out of contract and not used all season

Matt Simon - Out of contract and unlikely to fit into new plans

Mickael Tavares - Contracted but turns 34 in October

Jacob Tratt - Out of contract and not used all season

Robert Stambolziev - Out of contract and underwhelming this season

Riley Woodcock - Out of contract and barely used this season