2009-10: Dumba skated for the Edge School in Calgary and made his WHL debut with the Red Deer Rebels. Dumba played 16 games for the Edge School (including playoffs) and scored 5 goals with 7 assists. Taken in the 1st round (4th overall) of the 2009 WHL Bantam Draft, he played six games for Red Deer during the regular season and skated in two playoff contests.
2010-11: Dumba played 62 WHL games for Red Deer as a 16-year-old in his first full season and represented Canada Pacific in the 2011 U17 World Hockey Challenge. He scored 15 goals with 11 assists and was plus-24 with 83 penalty minutes as the Rebels finished first in the Central Division. In nine playoff games, Dumba scored 2 goals and was an even plus/minus. In six games at the WHC he had 12 assists and 2 penalty minutes.
2011-12: Dumba appeared in 69 game for Red Deer in his second WHL season and captained Canada's team at the 2012 U18 World Junior Championship. Dumba was the sixth-leading scorer amongst WHL defensemen with 20 goals and 37 assists and was -6 with 67 penalty minutes in 69 games. The Rebels missed the WHL playoffs, allowing Dumba to compete at the WJC. In seven games for third-place Canada he scored 5 goals with 7 assists and was plus-2 with 20 penalty minutes. Dumba was ranked 11th among North American skaters in Central Scouting's final rankings prior to the 2012 NHL Draft.
Dumba has the potential to be an electrifying player in the NHL, but he still needs to round his game into form before taking that next step. He is an energetic player who can score the big hit or the big goal, with his offensive game being more advanced than his defensive play. Dumba has good speed and uses it to rush the puck up the ice, and once in the offensive zone, he has a quick release and a good shot. His defensive game could use some work, but that may not be his ticket to the NHL, anyway.