Born in Les Planes d'Hostoles, Girona, Catalonia, Arnau was a youth graduate from La Liga powerhouse Barcelona.

[3] For the 2001–02 campaign, Arnau moved to Málaga for € 2.1 million, also playing second-fiddle until the departure of Pedro Contreras to Real Betis in 2003.

[6][7] Goitia was sold also to Betis in summer 2009, but Gustavo Munúa and Roberto Santamaría were also acquired, so Arnau was demoted to as low as third choice.

[8] After Munúa's departure, Arnau played the first game of the 2010–11 season – a 3–1 home loss against Valencia – as Rodrigo Galatto was not yet eligible.

[11] Arnau played his last match as a professional on 21 May 2011, coming on as a substitute for Willy Caballero in the last minutes of a 3–1 home defeat to his first club Barcelona,[12] with Málaga finally retaining its top-flight status.