Italiano, who started in the opening group matches against Turkey and the United States, suffered an adductor injury in training while Leckie sustained a hamstring strain during the USA match.
Both missed the 0-0 draw against Paraguay that put Australia into the Round of 32 on Thursday.
Leckie was a hero of Australia's World Cup in Qatar, scoring the winner in a group match against Denmark to seal their passage to the knockout phase.
The 35-year-old was selected for a fourth World Cup despite an injury-blighted season for Melbourne City.
Full back Aziz Behich, who slotted into the starting 11 in the absence of Italiano, said he was "gutted" for his City teammate.
"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," Behich told reporters on Friday, referring to Australia's pre-World Cup training camp in Florida.
"He left no stone unturned and it's a credit to him, it's not easy, not just physically but also mentally at his age."
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While Australia have depth to cover Leckie, Italiano's loss saw coach Tony Popovic shift regular left back Jordan Bos to the right to play as an inverted wingback against Paraguay.
Veteran full-back Behich started in Bos's place at left back.
The move proved effective as Bos was a standout in the scoreless draw, and his link-up play with Cristian Volpato down the right flank set up a number of promising attacks.
“It’s something we worked on during the week in our tactical session and it worked really well, and I think it worked well again last night," Behich said of the tactical switch.
"That’s why we got the result that we deserved."