He went on to appear in 126 competitive matches for Espanyol over six La Liga seasons before returning to his previous club in 2015, with whom he won three UEFA Champions League titles as backup to Keylor Navas.

However, Codina became the starter and Antonio Adán, a fellow youth graduate, the backup, and Casilla ended up playing only in the last game of the campaign, with an eventual relegation befalling.

On 20 January 2008, he made his debut in La Liga in a 1–2 away loss against Real Valladolid, coming on as a substitute for injured Iñaki Lafuente two minutes after half-time as the habitual starter Carlos Kameni was away on international duty.

[8] Barred by Kameni and new signing Cristian Álvarez, Casilla joined Segunda División B side Cádiz CF on loan for 2008–09.

On 1 February 2015, he was sent off against Sevilla FC for handling Iago Aspas' shot outside of the penalty area, and his replacement Pau López could not prevent an eventual 2–3 defeat.

[19] On 9 August 2016, Casilla was in goal for injured Navas in the 3–2 victory against fellow Spaniards Sevilla for the UEFA Super Cup, in Trondheim.

[29] On 17 January 2019, the 32-year-old Casilla moved abroad for the first time in his career and signed a four-and-a-half-year contract at Leeds United of the English Championship.

[42] On 29 August 2014, he was called up to full side manager Vicente del Bosque's 23-man squad for matches against France and Macedonia in September, as third-choice behind Iker Casillas and David de Gea.