Injured Reserve - What is it?

From Yahoo Fantasy Football help:

Across all fantasy games, Yahoo! requires a player to be on the real-life DL, IR, IL, etc. in order to qualify for a position there in the fantasy game.
Note: In professional football, if a player is placed on injured reserve, he is deactivated for the season and is not eligible to return. As a result, that player has no fantasy value for the current season.

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