And at just 19 years old, each has become a key contributor to a team with serious Stanley Cup Playoff aspirations.
Are both ready for full-time NHL duty? Whether they are or not, it's the only logical place they can continue their development.
Because both were drafted from Canadian Hockey League clubs -- Rielly from the Moose Jaw Warriors of the Western Hockey League, Maatta from the London Knights of the Ontario Hockey League -- they would have been required to go back to those teams.
Rielly had 54 points in 60 games with Moose Jaw last season, fifth among WHL defensemen, despite missing time for Super Series games and a stint with Canada at the 2013 IIHF World Junior Championship.
Maatta had 38 points and a plus-9 rating in 57 regular-season games, and 14 points in 21 OHL playoff games, third among OHL defensemen. He also missed time in the regular season to play for Finland at the World Junior Championship.
What those numbers show is that Rielly and Maatta have outgrown junior hockey. At 19 years old, they would be on the high end of the age scale, playing against a league comprised mostly of 17- and 18-year-olds. How would they get better in that environment?
Unfortunately, the only option for Rielly and Maatta would be a return to junior. The agreement between the NHL and CHL says players drafted from a CHL club are ineligible to play in the American Hockey League prior to turning 20. It's a rule that benefits the junior clubs by allowing them to keep their most marketable star players. However, in some instances it can slow a player's progression to the NHL. And Rielly and Maatta are two of those cases.
In speaking recently about another player from that 2012 draft class, forward Tom Wilson, Washington Capitals coach Adam Oates justified Wilson staying with the Caps by saying that while he would be a top-line player if returned to his junior team, the Plymouth Whalers of the OHL, it would be a step backward in his development.
"He can go back to junior and score goals and get assists and play 20 minutes," Oates said, "but he'd develop bad habits and they're not the goals he'd score here [in the NHL], not the assists he'd get here and they're not the situations or speed you'd get here."
That's the same situation the Maple Leafs and Penguins were faced with regarding Rielly and Maatta. Rielly has four assists and a plus-3 rating while averaging 18 minutes of ice time in eight games. Maatta has a goal and two assists in nine games while playing 15:03 per game.
Neither is in the top-four in ice time among their respective team's defensemen, but on their junior teams they would be the unquestioned No. 1 defenseman. However, there's the big fish/small pond theory again. Each player arguably was his team's top defenseman last season, received around 25 minutes of ice time per game and played on the top power-play units last season; how much more could they gain from that amount of ice time again this season in junior?
The Maple Leafs and Penguins are committed to keeping Rielly and Maatta only beyond the nine-game threshold that would start their entry-level contracts; Maple Leafs coach Randy Carlyle publicly left the door open for Rielly to be returned to Moose Jaw if his play drops off or if the Leafs think that's the better path for him.
But for now, the best path for Rielly and Maatta to become full-time NHL players is to remain in the NHL.
Follow Adam Kimelman at email@example.com. Follow him on Twitter: @NHLAdamK