Currently, the NFL's injured reserve rule restricts a player so designated from returning to play during the season that he's placed on the list. For further information on the rules concerning the contract status and compensation for injured players, see the Collective Bargaining Agreement (or CBA).

This represented a rule change which, I believe (I'm still researching this), was instituted when the current CBA was ratified.

Prior to that, the injured reserve rule distinguished between a player going on the list before the final roster was set (in pre-season) and after that. A player going on injured reserve in pre-season was, like now, lost for the season but a player going on the list during the season only had to sit out for a minimum of four games.

Apparantly, the NFL owners pushed for this rule change in an effort to police themselves. I've read that some, unnamed, owners (the NFL owners' meetings are private) felt that some teams were illegally using the injured reserve list to stash players who weren't really injured to circumvent the rules on roster limits (to gain a competitive advantage).