This afternoon the Leafs placed Tyler Bozak and Jake Gardiner on injured reserve. The NHL's collective bargaining agreement actually has two kinds of injured reserve and there are some important differences between them. Clark Aitken has covered the topic in some more detail in the past, but I'll give a brief run-down here and then explain what the Leafs might be thinking in this particular scenario.
The two types of IR available to NHL teams are regular IR and long-term IR (technically called the Long-Term Injury/Illness Exception to the Upper Limit). They overlap, so a player on LTIR is also on IR, but not necessarily vice-versa. If a player is placed on injured reserve, that means they're not eligible to play in an NHL game for 7 days. This move opens up a roster spot.
NHL teams are normally limited to 23 players on their active roster at a time, but a player on IR opens up a spot for another player. This has no effect on the salary cap.
Long-term injured reserve is a bit more stringent. If a player is placed on LTIR, they are not allowed to play in a game for 24 days and 10 games. The benefit to placing a player on LTIR is that, if you are already at the salary cap maximum, you can go over the salary cap by an amount up to the total of the injured player's salary and bonuses (it's a bit more complicated, but that's essentially how it works).
A player never has to be placed on any form of injured reserve. It's perfectly legal to keep an injured player on your roster if you don't want the extra roster spot or potential salary cap relief.
Both IR and LTIR can be retroactive up to the last game a player played in.
What does this mean for Bozak and Gardiner? Tyler Bozak last played a game on October 16th, exactly one week ago today. Technically he would be eligible to come off IR tomorrow, but by placing him on IR the Leafs are indicating that they expect him to be out for at least one more game. He could, therefore, be back in the lineup as early as Monday's game against Phoenix.
Gardiner last played on October 17th, meaning he will also be eligible to come back for Monday's game.
The Leafs have already used the extra roster spot created by placing Tyler Bozak on IR to call up Byron Froese. So why did they place Gardiner on IR but not call anyone up?
At the moment, with Gardiner and Robidas out of the lineup and Corrado in the AHL on a conditioning loan, the Leafs have just six defencemen on the roster (Phaneuf, Rielly, Hunwick, Marincin, Polak, Harrington). They play a road game against Montreal on Saturday. Teams generally prefer to have at least one extra defenceman around for road games, just in case someone gets injured during practice or warm-up. So they likely want Gardiner's roster spot available tomorrow.
Why not call-up anyone just yet? The Marlies play a game all three days this weekend, including one tonight. My guess would be that the Marlies want to get as many players into those games as possible, so one of the defencemen in the Marlies lineup tonight will likely be called up to the Leafs at some point tomorrow.
I wouldn't expect them to play in Saturday's game; they'll likely be kept around as pre-game injury insurance. Since Corrado is already taking up an NHL roster spot (players on conditioning loans still count against the 23-man roster limit), Gardiner being placed on IR would seem to indicate that the Leafs will use the call-up on a Marlie rather than ending Corrado's conditioning stint (which can last up to 14 days).
It's worth noting that while Bozak and Gardiner have not been placed on LTIR, they still could be at any point. But given the length of time they would have to be kept out of the lineup, and the fact that the Leafs still have some cap space to work with, there's no reason to place either player on LTIR until the last possible moment (since it can be done retroactively). If they do run out of cap space, the first move they'd likely make is to put Robidas on LTIR, before proceeding with Bozak or Gardiner only if they still need more space after that.