Valenzuela Jr. played collegiately at UNLV, before being drafted by the San Diego Padres in the 10th round (281st overall) of the 2003 draft. In 2003, Valezuela Jr. played 73 games for the Padres low-A team in Eugene, followed by playing  for the Fort Wayne Wizards; leading the team in hits (148) while hitting .295 in 2004. In 2005, he was promoted to Lake Elsinore (A+). In 2006, Valenzuela Jr. was picked up by the Chicago White Sox and played for the Single-A Kinnapolis Intimidators.

In 2011, Valenzuela Jr. hit .359 in 98 games with 387 at-bats and 125 hits, including 24 doubles, eight home runs, scored 40 runs, and drove in 61 RBIs and was named a Mid-Season All-Star for the Mexican League. Before being picked up by the Diablos, Valenzuela Jr. has spent 2012 appearing in 107 games, with 380 at-bats, 119 hits, 30 doubles, nine home runs, scored 37 runs, driven in 65 RBIs and hitting .314/.371/.464 for Yucatan.