Alonso, a native of Madrid, started his career as a kid in the Real Madrid development program. Over the years, he played for every youth team. He joined Real Madrid Castilla in 2008, a Segunda División B squad, and on February 22, 2008, he made his reserve team debut, playing the entire match in a 1-0 home loss to AD Alcorcón.
Alonso was first called up by the starting lineup, led by Manuel Pellegrini, for a La Liga encounter at Valencia CF on December 11, 2009. His debut came on April 4 of the following year when he replaced Gonzalo Higuain in the 90th minute of a 2-0 victory away to Racing de Santander after failing to make the final list of 18 players.
Alonso turned down Dougie Freedman’s offer of a new contract to sign a three-year deal with Italian club ACF Fiorentina in May 2013. Gus Poyet, the manager of Sunderland, revealed on December 30 that the player, who had made nine appearances for the club, will join on loan on January 1 and remain there through the remainder of the season.
