He is a skilled offensive forward whose biggest asset is his ability to change his skating direction very rapidly alongside with his clever passes. He is rather small for a professional hockey player which might be an issue when the play gets physical, though. Haapala first played as a 14-year old youth within the Tappara system in 2008. He played his first professional game with Tappara in the 2011–12 season. Early in the 2012–13 season he formed a great line with NHL-veteran Ville Nieminen and Aleksander Barkov as his linemates. Haapala had 13 points in 12 games counting from the start of the season, but after an injury, he lost the pace. He played most of spring 2013 for Tappara's junior team. In the 2013–14, 2014–15 and 2015–16 seasons, he was a regular in Tappara's professional squad, but injuries were holding his development back. There was notable progress in each season however. Haapala agreed to his third contract extension to remain in Tappara by signing a 3-year deal on 12 May 2016. In the 2016–17 season, Haapala finally made his breakthrough to international level. He scored 15 goals and had 45 assists in 51 regular season games and was awarded the Veli-Pekka Ketola trophy for leading the league in total points at the end of the regular season. In the postseason, he had 2 goals and 7 assists in 16 games as Haapala's team Tappara marched to their second consecutive championship. Haapala represented the men's national team in six games in which he had two goals and an assist. On 1 June 2017, Haapala was signed as a free agent by the Florida Panthers to a two-year, entry-level contract. He began the 2017–18 season with the Springfield Thunderbirds of the American Hockey League, but was later called up by the Panthers and made his NHL debut on November 22, 2017 against the Toronto Maple Leafs, assisting on a goal by Nick Bjugstad in a 2–1 victory. Haapala appeared in 5 games with the Panthers before returning to Springfield. On February 16, 2018, the Panthers assigned Haapala on loan to his former team, Tappara of the Finnish Liiga. After returning to attend the Panthers 2018 training camp, Haapala upon failing to make the opening night roster for the 2018–19 season, was reassigned to Europe on loan, joining Swiss outfit, HC Lugano of the NL, on October 5, 2018. In 21 regular season appearances with Lugano, Haapala posted 3 goals and 15 points adding a further 5 points in 3 playoff contests. As an impending restricted free agent from the Panthers, Haapala opted to continue his career in Europe, signing a two-year contract with Finnish club, Jokerit of the KHL, on 17 May 2019.
