Slager started his football career at RKC Waalwijk and signed his first professional contract with the club, keeping him until 2014.
In the first half of the season, Slager made nine appearances for the club, including scoring his first goal against Go Ahead Eagles on 31 August 2013.
[3] He started his first match for the Sky Blues against Barnsley in the third round of the FA Cup on 4 January 2014, which was won 2–1.
[7] Slager joined United Soccer League side LA Galaxy II on 18 February 2016.
[9] In 2013, Denzel was part of the Curaçao under-20 side for the 2013 CONCACAF U-20 Championship On 10 August 2021 Slager confirmed his relation with singer Famke Louise after they appeared together on the cover of the Dutch Cosmopolitan.