Funny enough, Mattinen was originally the Leafs' 6th round draft pick back in 2016 - the same year in which they drafted
Auston Matthews 1st overall. Mattinen was selected 179th overall that season. The former London Knights defenseman went on to play 3 more OHL seasons with London, Flint, Hamilton and Oshawa before playing two seasons for the University of Ottawa. Once his collegiate career ended, Mattinen enjoyed an 8-game cup of coffee with the AHL's Laval Rocket before taking his talents to Austria and Germany for the last two seasons.