Necas became a full-time player for the Carolina Hurricanes in 2019-20 after being their first-round pick (No. 12) at the 2017 NHL Draft. Three seasons later, he scored a Hurricanes-high 28 goals, and he signed a two-year, $13 million contract with the Hurricanes on July 29, 2024, to avoid an arbitration hearing. He had 53 points (24 goals, 29 assists) in 77 regular-season games and nine points (four goals, five assists) in 11 Stanley Cup Playoff games during the 2023-24 season.

Necas continued to blossom early in 2024-25. He was named the NHL's First Star for the month of November after leading the League with 22 points (seven goals, 15 assists) in 15 games to help the Hurricanes go 9-5-1, including 7-1-0 at Lenovo Center. Necas started the month by extending his point streak to 13 games, including an assist in 10 consecutive appearances, and then joined the Colorado Avalanche on Jan. 24, 2025, in a three-team deal involving the Hurricanes and Chicago Blackhawks that sent Mikko Rantanen and Taylor Hall to Carolina.

Necas signed an eight-year contract with Colorado on Oct. 30, 2025. He started his first full season for the Avalanche with an eight-game point streak (six goals, six assists) and got his 40th point in his 38th game since joining the Avalanche, the fastest since the franchise relocated to Denver from Quebec to begin the 1995-96 season. He was among the first six players named to Team Czechia's preliminary roster for the Olympic Winter Games Milano Cortina 2026, where he led the team in scoring with eight points (three goals, five assists) in five games.

He made the Hurricanes' season-opening roster in 2017-18 but played one game before being returned to Brno in the Czech Republic. He scored 17 points (nine goals, eight assists) in 24 playoff games to help his team win its league championship for a second straight season.

Necas played most of the 2018-19 season with Charlotte of the American Hockey League, but he did score his first NHL goal against the Tampa Bay Lightning on Oct. 16, 2018. He had two points in seven NHL games, then helped Charlotte win the AHL's Calder Cup with 13 points (five goals, eight assists) in 18 playoff games after he scored 16 goals and 36 assists in 64 regular-season games.

On the international stage, Necas led the 2018 IIHF World Junior Championship with eight assists for the Czech Republic and tied Casey Middlestadt (United States) for the scoring lead with 11 points. He served as Czech Republic captain in 2019 after the Hurricanes allowed him to compete in the tournament.

Necas signed a three-year, entry-level contract with Carolina on July 15, 2017.