On 7 February 2024, while playing with
Mikkelin Jukurit of the
Liiga, Helenius became the second youngest player in Liiga history to record a
hat trick, after
Tuomo Ruutu, and fifth to do so before the age of 18. He finished the season with 14 goals and 36 points in 51 games. Following the season, he was selected 14th overall in the
2024 NHL entry draft by the
Buffalo Sabres, with whom he signed a 3-year entry-level contract on 8 July. Helenius spent the entire 2024–25 season playing with the
Rochester Americans in the
AHL. During the 2025–26 season Helenius recorded 30 points (9-G, 21-A) in 34 games with the
Rochester Americans en route to being named to the 2026
AHL All-star Classic. On 15 January 2026 Helenius was called up to the Buffalo Sabres later making his NHL debut on 19 January. On 20 January, in his second career NHL game, Helenius scored his first
NHL goal, added two assists and was named the first star in a 5–3 win over the
Nashville Predators. ==International play==