Career
RKC Waalwijk
Slager started his football career at RKC Waalwijk and signed his first professional contract with the club, keeping him until 2014. In his first season at the club Slager made five appearance for RKC Waalwijk. In the first half of the season, Slager made nine appearance for the club, including scoring his first goal against Go Ahead Eagles on 31 August 2013.
Coventry City
On 3 January 2014, Denzel agreed an 18-month deal which would keep him with the Sky Blues until the summer of 2015.
On 18 February 2014 Slager had his contract cancelled by mutual consent. Orange County Blues
Slager signed with third-tier United States club Orange County Blues on March 27, 2015.
Slager went on trial at South Carolina Cambuur, but was unsuccessful of getting himself a contract. Los Angeles Galaxy II
Slager joined United Soccer League side Los Angeles Galaxy II on February 18, 2016.