no no-- sorry i didnt explain this well:
1) if a player gets hurt
in the game in your franchise, you can absolutely IR him by going to the injury screen in your franchise & pressing A @ the prompt to assign him to IR
2) if a player gets hurt in REAL LIFE & you just want to emulate that in the game, you can still IR him.... but you have to do so from in the editor
basically, the game wasn't designed to let you IR
healthy players-- you either need them to get injured in the game first or you need to use the editor