You hadn't heard of Youssouf Hersi either, right? Two years ago Tony Popovic pulled off a coup in securing the services of the former Ajax man, who helped Western Sydney to the A-League premiership and two successive grand finals before leaving for Perth Glory at the end of last season.
So has 'Popa' repeated the feat by luring another unheralded Dutch winger to Wanderland?
Tempering the excitement of those in red and black over the arrival of the 10-cap Netherlands international is the fact that Castelen is a year older than Hersi and has endured a horror run of injuries during his career.
A product of the ADO Den Haag youth academy, Castelen won the UEFA Under-21 European Championship with the Netherlands in 2006, playing alongside Urby Emanuelson, Klaas Jan Huntelaar and Ron Vlaar in the final.
He impressed for Feyenoord between 2004 and 2007, but it all went wrong upon joining Hamburg, where a succession of knee injuries restricted him to just 17 Bundesliga appearances in five years.
He managed two league games during a brief spell at Russian Premier League side Volga Nizhny Novgorod in 2013, before finally rediscovery form and fitness at RKC Waalwijk last season.
A total of 29 appearances, 28 of them starts, was a huge improvement on those lost middle years of Castelen's career, but he still missed six games between December and January due to (you guessed it) a knee injury.
When he was on the field, Castelen was a clear standout in a struggling team, with RKC finishing third from bottom and suffering relegation to the Eerste Divisie.
Of the 10 wingers and forwards to make the most appearances for the bottom three teams, Castelen was the second highest goalscorer with six strikes and boasted the second highest shooting accuracy of 61.29 percent, behind only Guus Huppers of Roda (nine goals) and NEC's Soren Rieks (64.52 percent) respectively.
He was the joint leading assist provider, setting up six goals for team-mates, created the most chances (38), topped the charts for passing accuracy in his own half (87.73) and the opposition half (78.28), as well as recording the best accuracy for crossing/corners at 36.84.
Although Castelen couldn't prevent RKC from dropping to the second tier, the Suriname-born veteran proved he's still capable of playing out a season and making a significant impact for his club, finally demonstrating the ability that once saw him tipped to make the Netherlands' 2006 FIFA World Cup squad.
If his knee holds up and his form from an Eredivisie relegation battle translates to Parramatta Stadium, the Red and Black Bloc could well have a new hero to cheer on, while Popovic will be lauded for another transfer market masterstroke.
But if Castelen's injury curse strikes again, the Dutchman is set to become very well acquainted with the Wanderers' treatment table. Is it a gamble worth taking?
Western Sydney Wanderers memberships are on sale now. Go to wanderland.com.au for more details and to sign up today!