The music producer - who has been on the panel of the show since it began - will instead become a ''mentor'' behind the scenes, as producers still want him involved.
A source told TMZ.com: ''There's some concern among the brass that Jackson is not well-suited for the mentor role ... but they feel they need him because he's the glue that helps keep the show together.''
Mariah is not thought to be a fan of Nicki's, although producers are delighted at the prospect of conflict between them both.
Currently it is hoped there will be four judges this year, with the third seat taken by a country music star and the fourth unknown.
'Fantasy' hitmaker Mariah is said to be keen for Lenny Kravitz to sign up to the show.
A source said recently: ''Mariah would love to have her dear friend Lenny Kravitz sit next to her on the 'American Idol' judge panel. She thinks he would be the perfect choice because he is extremely talented and knows music inside and out.''