Hong Kong-based Moreira went down on the Chris Waller-trained Regal Monarch during race four at Flemington and had to be assisted from the track.
He was stood down after a medical assessment.
The William Mullins-prepared Thomas Hobson, a 15-1 contender, will now be ridden by 18-year-old Ben Allen, the Irish trainer confirmed.
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"I've spoken to him (Allen) already," Mullins said in a televised interview. "I'm happy enough with it."
The Melbourne Cup, the world's richest two-mile handicap, gets underway at 0400 GMT.
(Reporting by Ian Ransom; Editing by Peter Rutherford)
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