Trejo was signed by the Detroit Lions as an undrafted free agent following the 2001 NFL draft.
During his rookie season in 2001, he made a career-long reception of 20 yards against the Dallas Cowboys on January 6, 2002.
Trejo served on special teams with 23 tackles for third on the Lions, 5 of those coming at Arizona, on December 8, 2002.
In 2003, Trejo played in all 16 games, mostly on special teams, and saw limited action on offense at the end of the season.
[1] Trejo made his first career start against Green Bay on October 17, 2004.