It's being called Australian football's worst kept secret, but perhaps it's just it's worst-timed announcement. 

The news that Arnold will leave his role at Sydney FC at the end of the season, to become Socceroos coach following Bert van Marwijk's spell at the 2018 World Cup, could barely come at a worse time for Sydney FC.

The champions-elect have now suffered back-to-back losses against fellow A-League Championship-contenders Newcastle Jets and J-League side Kashima Antlers in the Asian Champions League (ACL).

While Sydney FC are still favourites to take out a second successive Premiership / Championship double, Arnold's men are pinning their hopes on a dramatic ACL revival after a disappointing 2-0 loss to the Antlers left them bottom of their group.

Kashima are top with seven points, a two-win gap that seems insurmountable for the Sky Blues, however K-League side Suwon Bluewings are only one win above Sydney in second place, with Chinese side Shanghai Shenhua third with three draws.

Sydney next face Kashima Antlers in Japan in a tough task that could see Sydney sink further behind in a dismal ACL campaign for the ambitious club. Whether they'll be able to raise their level, given the poor timing of the announcement, remains to be seen.

Arnold was Socceroos coach between December 2006 and November 2007, presiding over a quarterfinal loss to Japan at the 2007 Asian Cup, and a win percentage of 33.33% (three wins, one draw, five losses).