Wanderers coach Tony Popovic gutted the side that lifted the AFC Champions League trophy in December, with 18 players having packed their bags since the end of last season.
Castelen said the long pre-season meant the host of new signings - including midfielders Mitch Nichols and Andreu and new marquee striker Federico Piovaccari - will have enough time to gel with the squad.
"That's the beauty of the A-League, you have a very long pre-season," one-time Netherlands international Castelen said.
"So if you get your squad together very quickly you can build to something for the first game."
The Wanderers finished second bottom in the A-League last season after ACL travel commitments left the squad exhausted.
A number of key players, including striker Tomi Juric, midfielders Iacopo La Rocca and Mateo Poljak, defender Matthew Spiranovic and veteran goalkeeper Ante Covic have moved on, leaving Popovic with an almost blank slate upon which to build a new team - something he did just three years ago.
Castelen, 32, said effectively replacing those players would be critical for the squad's revival.
"Of course, I'm a player, I hope we get good players to have a good season," he said.
"But the main concern is for the bosses upstairs to bring good players in so we can have a good squad that can compete."
The Red and Black have gone some way towards achieving that with the arrival of ex-Sampdoria and Eibar striker Federico Piovaccari.
Castelen said the team's new marquee man was an unknown quantity but would hopefully lead the line with aplomb.
"The coach and the directors, they do good research so they will know his quality," he said. "He's a striker so he should bang in the goals for us.
"But I think all the players have to give their contribution to the club."
The Wanderers will play Mt Druitt in a pre-season trial on Thursday night, at which Piovaccari will be introduced to the crword, before the squad jets off to Coffs Harbour for camp and another trial against Central Coast Mariners on Friday 7 August.
It's first competitive game will be in the FFA Cup Round of 32 match against Brisbane Roar on Tuesday 11 August.
Western Sydney Wanderers squad
Goalkeepers: Dean Bouzanis, Andrew Redmayne
Defenders: Jonathan Aspropotamitis, Shannon Cole, Shayne D'Chuna, Brendan Hamill, Scott Jamieson, Scott Neville, Nikolai Topor-Stanley
Midfielders: Andreu, Kearyn Baccus, Mitch Nichols, Jacob Pepper
Attackers: Mark Bridge, Romeo Castelen, Golgol Mebrahtu, Federico Piovaccari, Brendon Santalab, Jaushua Sotirio
NYL Contracted Players: Alusine Fofanah, Daniel Alessi, Liam Youlley
