Rosales began his career with Newell's Old Boys. His play with Newell's attracted the interest of top European clubs and in 2004 he joined Dutch top side Ajax. While with Ajax Rosales appeared in 63 league matches scoring 6 goals. In 2007 Rosales left Ajax returning to Argentina to play for River Plate for a fee of 1.8 million euros. In 2008 he was part of the squad that won the Clausura 2008 tournament, the club's first title in four years.
In January 2011 it was reported that he would be joining Querétaro F.C. of Mexico's Primera División. However, he was not signed by the Mexican club. In late February 2011 he went on trial with Seattle Sounders FC of Major League Soccer in the United States. On 18 March 2011, following a successful trial, the club officially signed Rosales. During Rosales' first season with Seattle, he scored 5 goals in 26 appearances. Additionally, he assisted on 13 goals which was third overall in the league. His 13 assists also set a club record for number of assists by a single player in a single season. Rosales won the MLS Newcomer of the Year Award for the 2011 Major League Soccer season and was subsequently signed to a new, multi-year deal on 13 December 2011.