With Christian Dvorak's return and the fact that Rafaël Harvey-Pinard is not seriously injured, the Canadiens had 24 players on their roster, while the lineup, including reserves, can only have 23.
This is where the importance of having two goaltenders and not three becomes evident, as it eliminates the possibility of having an extra forward or defenseman.
So, the Canadiens had a decision to make, and Gustav Lindstrom was ultimately assigned to the Laval Rocket.
"The Canadiens have loaned defenseman Gustav Lindström to the Laval Rocket."
This means that Lindstrom will have only made a brief stop in Montreal, and there is currently no surplus defenseman with the team.