Everyone has an IR slot. You can use your lost if after the NFL Injury report you have a player that is either Doubtful or Out.
If before the games if your player changes status and moves to Questionable you must drop a player to move that player back to your active roster. I and the other owners help self-police this to be fair to everyone, so if you not catch it yourself, I will always send you a PM or an email to alert you to make the change.