Määttä began the 2012–13 season with two points in his first four games, though his offensive numbers began to pick up about a month into the season, as the Knights went on a 24–0 streak between 1 November and 1 January. In late December, Määttä joined Finland for the 2013 World Junior Championships. Määttä was outshined by fellow Finnish defencemen Rasmus Ristolainen and Ville Pokka as Finland finished fourth in its pool, sending the team to the relegation round. Finland easily won its two relegation matches to stay in the top tier, though a seventh-place finish in the ten-nation tournament was nonetheless considered a disappointment.