 On 11 May 2017, club president Tommaso Giulini akazivisa Cossu aizodzokera Cagliari nokuda 2017-18 mwaka. Akaita mutambo wake wechitatu wekirabhu, uye 247th mutambo wakazara weCagliari, mukukundwa 21 kure kuMilan. Musi wa 8 Nyamavhuvhu 2018 akatamba mutambo wake wekuperekedza mumutambo wehukama neAtlético Madrid, akachinjwa nemhemberero paminiti yechinomwe yemutambo uye akabva pamudyandigere semutambi. 