Shortly after suffering the injury, Marner was placed on Long-Term Injured Reserve (LTIR), retroactive to December 3rd, and given a timeline of approximately 3-4 weeks for recovery.
Per the NHL's rules relating to LTIR, a player must miss a minimum of 10 games and 24 days before they become eligible to return to the lineup. Originally, it was thought that Marner would not be eligible for return until January 2022 due to the Leafs having several games postponed over the last week, but The Star's Kevin McGran has revealed why that isn't the case.
According to McGran, if Marner's shoulder is feeling good and he is cleared by the team's medical staff, he could play in the Leafs' next game on December 27th against the Columbus Blue Jackets, even though he has only missed 6 games.
At this point, it appears somewhat unlikely that Marner will return on December 27th, as he will not be able to practice or train due to team facilities being closed until after the Christmas break, but until the team says otherwise, it remains a possibility.