Nigeria have already lost forward Victor Boniface and midfielder Wilfred Ndidi due to injury, while striker Kelechi Iheanacho has had a muscle problem and is expected to arrive in the Ivory Coast on Saturday.
Real Sociedad striker Sadiq had been a doubt for some time with his knee injury, but was officially ruled out two days before Nigeria open their tournament against Equatorial Guinea in Abidjan on Sunday.
Onuachu, 29, has spent this season on loan at Turkish side Trabzonspor from Southampton, where he has scored nine league goals in 15 starts. He last played for Nigeria in March 2023.
Nigeria will also face hosts Ivory Coast and Guinea-Bissau in Group A.
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