Nečas led the 2018 World Juniors with 8 assists and tied for the scoring lead (with Casey Mittelstadt of the United States) with 11 points, helping the Czechs to a fourth-place finish. He was again named to the senior team for the 2018 IIHF World Championship, finishing with 5 points in 7 games. The Hurricanes lent Nečas to the 2019 World Juniors, where he served as captain of the Czech team.