The All Blacks have called up Bath's new recruit Stephen Donald and wing Hosea Gear.
Slade, who had become first choice fly-half after the All Blacks lost Dan Carter, suffered a groin injury tear in Sunday's 33-10 quarter-final victory over Argentina.
Muliaina fractured his shoulder and failed to re-appear at half-time in what was his 100th - and now last - Test appearance for the All Blacks.
New Zealand coach Graham Henry said: "It's obviously very disappointing to have tournament-ending injuries.
"Hosea and Stephen will join the team on Tuesday morning. That's the reality of the situation."
The All Blacks campaign has been beset by setbacks, with star fly-half Carter ruled out with a groin problem and captain Richie McCaw playing through a foot injury.
Cruden has leapt from being outside of the World Cup squad to being the All Blacks' first choice fly-half a week before their crunch semi-final against Australia.
The 22-year-old enjoyed a confident introduction against the Pumas when Slade was hauled off injured.
"Just keeping Aaron Cruden off his skateboard has been the major thing," said All Blacks coach Graham Henry.
"Last week he was skateboarding around Palmerston North, having a couple of beers and watching us play. Now he's the number one 10 in the country.
"It's a big change and a major challenge for him, but I thought he handled the situation particularly well at the weekend when he went on."
Henry revealed the strapping on Cruden's knee was to cover a graze he suffered when coming off his surfboard.
Donald, who will join Bath after the World Cup, was out fishing when the news came through that he had been called into the squad to provide fly-half back-up.
Gear was one of the All Blacks' surprise omissions when their World Cup squad was originally announced.
"Hosea Gear and Stephen Donald join the team tomorrow morning," Gatland confirmed.
"The reason why they're joining the team tomorrow morning is so that we can say goodbye to these guys correctly. That's the reality of the situation.
"It's not that sensitive, but that's the way it is."
The All Blacks should have Israel Dagg and Richard Kahui available for the Wallabies showdown.
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