Roy, 21, made four appearances in the preseason and contributed two assists.
Last season, the six-foot center played in 23 games for the Canadiens, scoring four goals and dishing out nine points.
The 21-year-old Mailloux had four preseason appearances and a plus-6 rating while recording an assist. The 6-foot-3 defenseman played in 72 games with the Rocket last season, scoring 47 points and 14 goals.
While Emil Heineman and Kapanen excelled in training and were added to the roster, Joshua Roy faltered and fell. Even though they might not wear uniforms for every game, they are still staying with the Canadiens while he isn't.