In 2003, Valdez rejected the chance to play for the Germany national under-21 team in favour of Paraguay, having been invited to trial with Germany U21 by coach Jürgen Kohler, he was eventually was selected to represent the Paraguay under-20 national team at the 2003 FIFA U-20 World Cup in the United Arab Emirates. Prior to this, Valdez had been unheard of from his under-20 teammates and coach, Rolando Chilavert, brother of José Luís Chilavert, and was selected due to his presence in German football. Valdez did not know the names of any of his teammates from the squad and eventually met them on the airplane. Paraguay's U20 squad had already finished third at the 2003 South American Youth Championship, and eventually qualified for the FIFA U20 World Cup. Valdez was one of three players based outside of Paraguay, alongside Dante López at Israel's Maccabi Haifa and Víctor Hugo Mareco at Italian club Brescia. The squad also featured players such as Édgar Barreto, Julio dos Santos, Ernesto Cristaldo and Erwin Avalos, with several of these players going on to represent Paraguay U23 at the 2004 Summer Olympics. Valdez featured in all three group-stage fixtures against United States, South Korea and Germany, scoring Paraguay's second goal against Germany in their 2–0 victory, which resulted in Paraguay finished in second position of their group with six points. During the competition, Valdez humorously revealed to Paraguayan newspaper ABC that in the fixture against Germany U20, he faced his Werder Bremen teammates Christian Schulz and Alexander Walke who made a bet with him that if he would score a goal then one of his teammates would owe him a dinner and would pay for his meal. Valdez then featured in Paraguay's round of 16 clash against Spain U20, where which Paraguay were beaten 1–0.