 Born in Bilbao, Biscay, Casas came through the ranks of Athletic Bilbao and made his professional debuts during 2004–05.He gained first-choice status the following season after the departure of Asier del Horno to Chelsea, managing 24 league appearances and scoring twice during the campaign, at Getafe CF (1–1 draw) and Deportivo de La Coruña (2–1 win).Ironically, Casas was left out of the first-team squad at the start of 2007–08, after Athletic re-signed del Horno on loan from Valencia CF.