Slade suffered a groin injury in the All Blacks' 33-10 victory over Argentina in the quarter-final at Eden Park yesterday, and was replaced by Aaron Cruden, while Muliaina injured his shoulder in the match, which was his 100th test.
"Mils fractured his shoulder. This makes it impossible to carry on so we have withdrawn him from the tournament," All Blacks' doctor Deb Robinson told reporters.
"As for Colin, there's a partial tear in his left adductor. Just with the timing of what that is, there would not be enough time for him to rehabilitate to take any further part in the tournament."
Cruden was himself called into the All Blacks squad after first-choice flyhalf Daniel Carter tore a tendon in his groin during the pool phase and was ruled out of the tournament.
"We've lost the first two blokes chosen and that's a setback, that's the reality of it," All Blacks coach Graham Henry said.
"We have a group of players who have been on standby. we didn't announce that group and we won't announce that.
"We've got less players here than overseas - have we got the depth? We'll find out next week."
The 27-year-old Donald last played for the All Blacks as a replacement against Scotland in 2010, while Gear, 27, was widely considered the unluckiest player to miss out on the All Blacks' World Cup squad, which was confirmed by Henry.
"Hosea was very close to getting in the squad initially... very very close, it was very tight," he said.
Donald and Gear would join the team in Auckland on Tuesday.
"Obviously Colin is very disappointed. Very disappointed to be leaving the group at this stage of the tournament or any stage.
They can't stay with the team... the replacements join the team tomorrow morning. The reason for that is so we can say goodbye to the fellas properly."