A 5’10, 169-pound winger, Aaltonen was left unsigned by the Anaheim Ducks, who drafted him out of the SM-Liiga in the sixth round in 2013. Aaltonen scored twice in his first game at the World Juniors in his draft season before breaking his ankle in the tournament.
After five seasons in SM-Liiga, Aaltonen joined Vityaz in the KHL on a one-year contract for the 2016-17 season and put up 44 points in 59 games — good enough for 19th in points and first among players 24 and under (in points-per-game, Aaltonen ranked 30th overall and sixth among U24).