Fuentes was drafted by the Seattle Mariners in the 25th round of the 1995 amateur draft, and was signed by Mariners' scout Rodney Davis. Through the first four years of his Major League career, Fuentes had a record of 8-8 with a 4.04 ERA, recording only four saves in 163 games. However, in 2005, Fuentes took over the closer role for the Rockies following the season-ending shoulder surgery of right-handed pitcher Chin-hui Tsao. Fuentes had a successful season and was named to his first career All-Star Game, but did not appear in the 76th Midsummer Classic. He became the first reliever and only the third pitcher overall to represent the Rockies, joining starting pitchers Mike Hampton (2001) and Shawn Chacon (2003). His 31 saves matched Dave Veres (1995) for the third highest single season total in franchise history, trailing only José Jiménez (41 in 2002) and Chacon (35 in 2004). In 2006, Fuentes earned 30 saves while posting a 3-4 record and a 3.44 ERA.He is very close with his cousin Daniel Fuentes in Eglin AFB, Florida.