Villarreal was signed by the Diamondbacks as an amateur free agent in 1998. The next year, he made his professional debut with the Rookie League team the AZL Diamondbacks of the Arizona League. In 1999, Villarreal placed second on the team in innings pitched (64.1), games started (11), and strikeouts (51).\n', '

Villarreal began the 2000 season with the Triple-A Tucson Sidewinders. After being demoted to the Single-A South Bend Silver Hawks, Villarreal went 1–0 with a 1.66 earned run average. He was sent back down to Tucson in the Rookie League shortly before being sent up to Single-A High Desert, where he finished the season. In High Desert, he went 0–1 with a 2.70 ERA.\n', '

Villarreal played the entire 2001 season with the Double-A El Paso Diablos, where he logged a 6–9 record with a 4.41 ERA and 108 strikeouts. He began 2003 in El Paso, where he compiled a 3.74 ERA with a 6–3 win-loss record. On July 11, Villarreal was sent back up to the Sidewinders, where he went 3–3 with a 4.36 ERA. In the season combined, Villarreal recorded a Minor League career high 125 K\'s (85 in El Paso, 40 in Tucson).\n', '

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