With the Victory having believed they were on the verge of locking in Kewell, Postecoglou had planned to use the club's last overseas-born player slot to boost their defence.
Victory leaked more goals than even wooden-spooners Gold Coast last season.
Instead, Kewell's decision to return to England for family reasons means Postecoglou will most likely be chasing a class attacking player from overseas.
"I thought I had it all figured out," Postecoglou told reporters on Monday.
"But obviously we've got to make a choice now about where we fill that last foreigner spot.
"I was leaning towards a defender, to be honest, with Harry there.
"But now with Harry out of the picture, we might have to look further up the park and try to find an Australian solution, so to speak, for the defensive side."
Postecoglou said Kewell's departure did not make him regret his decision to cut Costa Rican star Carlos Hernandez.
The pair combined for 18 of the Victory's 35 goals last season.
Postecoglou said the disappointment was he had expected Kewell, who steadily improved after a modest start in his sole A-League season, to truly find his feet in the coming campaign.
He said while the club had a duty to fans to find a replacement who could deliver similar on-field excitement and class, Kewell's off-field impact was unique.
"He had probably the highest profile of any Socceroo in the last 30 years, maybe ever," Postecoglou said.
"That's hard to replicate in an Australian player."
He was not aware of any high-profile, available Socceroos that could meet the team's needs, meaning their new marquee player would almost certainly be a foreigner.
The ex-Brisbane Roar dual championship coach said the Victory had considered contingency plans in case Kewell left but it had still taken him time to digest the news when he received it on Saturday night.
"I had a couple of bottles of red, then a couple more. I (still) stayed awake all night but at least I had a good time," Postecoglou said.