Määttä was the first overall selection in the 2011 Canadian Hockey League Import Draft, and played for the Finnish junior team at the 2011 IIHF World U18 Championships and the 2011, 2012 and 2013 World Junior Ice Hockey Championships. Known for his shot-blocking ability and hockey IQ, Määttä was named to Finland's roster for the 2014 Winter Olympics as a 19-year-old.