Nečas first played internationally with the Czech U-17 team at the 2015 World U-17 Hockey Challenge, where he served as captain and recorded five points in five games, leading his team in scoring. He again was captain for the Czechs at the 2016 Ivan Hlinka Memorial Tournament, an invitational tournament, where he finished seventh overall in scoring with six points in four games, helping the team win gold. He later made his debut in an International Ice Hockey Federation (IIHF) tournament at the 2017 World Junior Ice Hockey Championships, where he had three points for the Czechs. He also participated at the 2017 IIHF World U18 Championships, serving as captain of the Czech team, and had three assists in five games as the Czechs lost in the quarter-finals.