Seabrook made his professional debut after completing his fourth and final WHL season, being assigned to the Norfolk Admirals, the Blackhawks' American Hockey League (AHL) affiliate, for the final three games of the regular season and six post-season games. Seabrook cracked the Blackhawks' line-up his first year out of junior in 2005–06 and recorded five goals and 32 points as a rookie. In his third NHL season, in 2007–08, Seabrook matched his rookie total with 32 points while tallying a personal-best nine goals. He began the season on the top-defensive pairing with Duncan Keith.