Australia sealed second place in Group D, and with it, a round-of-32 clash with the Group G runners-up in Dallas on July 1 after drawing 0-0 with Paraguay at Levi's Stadium on Thursday (Friday AEST).
They will learn their opponent at 1pm Saturday (AEST) when Egypt (four points) will play Iran (two) at the same time Belgium (two) face New Zealand (one).
Australia are down to 24 players for the knockouts as they cannot replace Italiano and Leckie, who left camp on Friday for Austria and Melbourne respectively.
Italiano (groin) played against Turkey and the United States but was injured at training, while Leckie (hamstring) pulled up sore in the defeat to the US.
Behich and goalkeeper Patrick Beach were disappointed for the injured pair.
"I saw first hand this year what he had to do to get back on that pitch for us at Melbourne City, and then what he did in Sarasota ... to prove himself to the boss and the coaching staff," Behich, a Melbourne City and three-time World Cup teammate of Leckie, said.
"... We're all gutted for him because we want him to stick around because we know what he can give us as a team, and even when he's not playing his experience and presence around a lot of the players."
Both losses are a blow to Australia's depth as the tournament continues.
Italiano's absence could mean Jordan Bos remains at right-back - where he dominated against Paraguay - with Behich staying on the left.
In the last 32, there are potential storylines everywhere - the obvious Australia-New Zealand rivalry or facing Iran for the first time since they dealt the Socceroos MCG heartbreak in 1997.
Behich is desperate to claim a knockout win after Australia fell desperately short against eventual champions Argentina in the round of 16 in Qatar.
Behich echoed the sentiments of teammate Jackson Irvine, who urged the squad to "break that next barrier" of winning a knockout game.
"Obviously second time out of the group now, but the last one left a bit of a sour taste in my mouth," Behich said.
"It's still got that burning fire in the belly. We've always said we want to do something special with this group and why can't it be us?
"And I think we've got a great opportunity. We've created this opportunity from our hard work to be here - it wasn't given to us.
"Everything's in our hands leading into this game. So (I'm) full of confidence that ... this group can do something special this campaign."
The Socceroos had a light recovery session on Friday as they soak up their achievement.
"For me, you need to enjoy what we did last night, because these moments don't come around often, especially at this level," Behich said.
"It's the pinnacle of football, you're up against the best in the world, and we did something special coming from Australia, so we really need to enjoy that.
"But once this week starts with our training sessions, we need to put that aside and really focus on what's next and I think that's important."
Australia won't play again until July 3 in Dallas, and won't relocate from their Bay Area base until July 1.
"We've got eight days to recover, which helps our younger players to get over this euphoria of the win - and then we need to work hard, and we're a chance," coach Tony Popovic said.
