Määttä joined a London team primed to make a run for the J. Ross Robertson Cup. The Knights won their first five games by a combined score of 31–10 and did not look back. London ended the regular season with the best record in the OHL (49–18–0–1) and the top defence in their conference. Along the way to posting a +25 plus-minus rating for the team, Määttä paced the team in defensive scoring with 32 points in 58 games, despite missing 6 games in the middle of the season to play for Finland at the 2012 World Junior Championships. Seeking to improve on Finland's sixth-place finish in 2011, Määttä was expected to be a key contributor for his team, but in the tournament opener he was concussed on a hit by Canada's Boone Jenner and missed the remainder of the tournament. Finland ultimately finished fourth without him. In the OHL playoffs, Määttä continued his strong play, only being held pointless in just five of London's 19 post-season games. As the Knights rolled through each series, losing only three games on their way to their first title since 2004–05, Määttä finished tied for the team lead in scoring with 23 points in 19 games.