He joined the youth ranks of AFC Ajax in January 2013 from FK Příbram. He made his professional debut at Jong Ajax on 13 March 2015 in a Eerste Divisie game against VVV Venlo. On 15 August 2015 he made his first appearance in the Ajax first team against Willem World War II Černý was called up to the Czechoslovakian Republic under-21 team in August 2015, at the age of 17.
This call-up drew media attention as he would become the youngest player to represent the team, if he played in the matches against Malta or Latvia in early September.