On April 27, 2003, the Indianapolis Colts signed Brackett as an undrafted free agent after he was not selected in the 2003 NFL Draft.
Brackett competed for a roster spot as a backup linebacker and special teams player during training camp in 2003. He impressed the coaching staff with his ability covering kickoffs. He made his professional regular season debut during the Indianapolis Colts' season-opener at the Cleveland Browns. He finished their 9-6 loss with one solo tackle. On October 26, 2003, Brackett collected a season-high six combined tackles (five solo) during a 30-21 victory against the Houston Texans in Week 8. In Week 12, Brackett made three solo tackles and made his first career sack as the Colts defeated the Buffalo Bills 17-14. He sacked Bills' quarterback Drew Bledsoe for a nine-yard loss during the second quarter.
During a 17-month span, starting in 2003, Brackett lost his mother, father, and brother.