At a top speed of 33.8 km/h, he was deemed by Fédération internationale de football association as the fastest player of the 2014 Fédération internationale de football association World Cup. Born in Heredia, Costa Rica, Díaz started his career at local side Herediano before moving abroad to join Polish side in January 2008. He moved on to Belgian club Club Brugge in September 2010 and was snapped up by German Bundesliga outfit FSV Mainz in summer 2012.
In 2015, after three years in Mainz, Díaz joined Darmstadt 98, a newly promoted team for the 2015-2016 season. career Júnior Díaz made his first appearance for the senior Costa Rica national football team in a friendly against China on 7 September 2003.
Díaz made one appearance at Copa América 2004, entering as a second-half substitute against Chile. Díaz also appeared in one match at the 2005 CONCACAF Gold Cup finals. goals.