Guzmán was originally signed by the Kansas City Royals as an amateur free agent on March 4, 1999 but his signing was voided and he signed with the Chicago Cubs on November 12, 1999. In 2000, he went 1–1 with a 1.93 ERA for the La Pradera club of the Venezuelan Summer League. In 2001, he pitched for the Boise Hawks, going 9–1 with a 2.23 ERA, 19 walks and 63 strikeouts in 77 innings pitched. He was second in the Northwest League in ERA, trailing Jesse Foppert, and he led the league in wins. He made the league All-Star team. He was rated the #4 prospect in the league according to "Baseball America".