[7] Kapustka finally made his senior debut for Leicester in a 2–1 FA Cup third-round win at Everton on 7 January 2017, coming on as an 84th-minute substitute for Marc Albrighton.

[8] He made his full debut in a 3–1 FA Cup fourth-round replay win over Derby County on 8 February 2017,[9] playing in a central attacking role behind striker Ahmed Musa.

On 7 September 2015, Kapustka made his debut appearance for the Poland national team in a match against Gibraltar, substituting Jakub Błaszczykowski.

Following the incident, he made a phone call personally to Poland national team manager Adam Nawałka, to ask for his forgiveness and ask that he not be excluded from the squad for the upcoming tournament.

[14] Kapustka was eventually selected for Poland's Euro 2016 campaign, and produced a standout performance in the team's opening match victory over Northern Ireland.