During the 2021–22 season, he made his debut in European competitions, contributing five goals in the UEFA Europa Conference League across qualification rounds and the tournament proper.

[9] He scored his first two goals for FCK on July 29, 2021, when he was substituted at half-time in a match against FC Tarpedo-BelAZ Zhodino in the Conference League.

[11] Initially confined to a substitute role at the beginning of the season, he carved out a place for himself in the squad due to the poor form and injuries of usual starter, Duván Zapata.

[15] Højlund made his debut on 3 September, replacing Anthony Martial in the 67th minute of a Premier League match against Arsenal, which ended in a 3–1 defeat.

[16] On 20 September, Højlund scored his first goal for the club in a 4–3 away defeat against Bayern Munich in the opening match of the 2023–24 Champions League.

This made him the youngest player to score in six consecutive Premier League games ahead of Joe Willock's previous record.

[27] In the final on 25 May, Højlund won his first trophy with the club as he featured as a late substitute for Marcus Rashford, helping secure a 2–1 victory over local rivals Manchester City.

[31] On 19 October, Højlund scored his first league goal of the season in a 2–1 home victory against Brentford, ending Manchester United's winless run of five games in the process.

This made him one of three United players to have scored nine goals in Europe at 21 years of age or below, the others being Marcus Rashford and Wayne Rooney.

[36] On 23 March 2023, he made his first international start, scoring a hat-trick in a 3–1 victory against Finland during the UEFA Euro 2024 qualifying, his first goals for his country.