The winner of Saturday night's game will play the loser of the two-leg major semifinal in which Brisbane Roar hold the advantage after their 2-0 first-leg win at home over minor premiers Central Coast Mariners. The Roar travel to Gosford for Sunday's return leg.
If the Phoenix win in Perth, they plan to remain in Australia to prepare for the preliminary final, likely to be in Gosford.
Meanwhile, Melbourne Victory coach Jim Magilton has decided not to seek another season with the club, opting to return to his native Northern Ireland rather than be part of the Victory's selection process as they seek to appoint a new coach in what has become a poisoned chalice at the big-spending club.
There will also be new coaches at the Heart and Sydney FC next season after John van't Schip and Vitezslav Lavicka's decisions to move on. They join Miron Bleiberg, who was pushed out the door at Gold Coast United, and Rini Coolen, who suffered the same fate at Adelaide United.
Perth's Ian Ferguson has signed on for another two years, while Gary van Egmond (Newcastle), Ange Postecoglou (Brisbane), Graham Arnold (Central Coast) and Herbert are not going elsewhere in a hurry.