Toni was coaching this season in the UEFA Champions League with CFR Cluj when the Romanian club fired him a month ago.
Toni previously worked in Portugal at Trofense, Vitoria de Setubal, Naval 1 de Maio and Estrela of Amadora.
Belenenses hasn't won since the second game of the season, on Aug. 24.
Despite its miserable showings, Adu has only played in three league games for Belenenses. The Lisbon club is his third European club in three seasons at which he has seen limited playing time.