In August 2007, Casilla was acquired by RCD Espanyol on a free transfer. 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. That month, news surfaced that Arsenal were interested in acquiring the young Spaniard's services.