The NFL is investigating why quarterback Tony Romo wasn't listed on the official injury report despite his admission that he was playing with broken ribs.
In 2009, the NFL fined the Jets $125K for not disclosing a Brett Favre injury.
What injuries have to be reported? And why? This policy seems like it could pose a threat to injured players as it draws attention to players and body parts that could be targeted by opponents during game time.
What is the reasoning here?
The injury report dates back to 1947, after a player unexpectedly did not play in a game, which ended up having significant impacts to wagering over the game. As such, the injury report serves the purpose of ensuring no funny business occurs related to injured players (not only that players who are expected to play do, and who are expected not to do not; but that all parties have equal knowledge, or at least as much as feasible. Otherwise insider information could give some parties a significant edge - not that the NFL directly cares about this, but they do care about their image not being tarnished by accusations of teams hiding information to alter the betting line.
As far as player safety, targeting an injured player is never legal. I doubt injury reports help in this manner; if a player is sufficiently injured such that additional hits may injure him further, it likely will become obvious to players on the field based on how he comports himself and how he reacts to hits.
The USA today piece linked above has more information on the subject and is quite thorough.
All injuries that could hamper a player's play or their ability to play must be reported according to NFL guidelines. The NFL has also added an extra level to injury reports by measuring a player's participation in each practice.
Do coaches report everything? No way. Look at Andrew Luck this year. You don't think if teams knew he had busted ribs he wouldn't be blitzed half the game?
What do teams do? Well most of the time injuries will surface from other sources. There are beat writers that will mention someone was stretching their hamstring a lot and not practicing. Something major is very hard to hide. It is even a lower chance that there is a significant injury that can be hidden but the player will also play.
Joe's answer is right on that the need for an injury report stemmed from gambling and insider activity. Imagine massive wagering based on knowing your first two quarterbacks were out - as the team janitor. Team janitor could just sell this information too.
What I don't think is stressed enough is the impact of revealing injuries on a weekly basis. It makes keeping a healthy team even more important. It is a true double-whammy. Your player doesn't play as good because he is hurt PLUS the other team plays (him) differently because of the injury.
The easiest example I can think of is a WR dealing with ankle or foot issues. Whatever leg it is on you know they will have trouble pushing off with that foot - limiting their route tree. You also know that a quick slant will be harder to run. What do most teams do in this situation? Hard-man press on this WR. Perfect example? Sherman on Dez Bryant coming off broken foot. There is no way Sherman plays Dez that tight (Dez is a big strong boy) if Dez were totally healthy. The Cowboys even trying to throw to him at all was laughable and might have cost them the game. Not only was Sherman smothering him but it took him 2-3 seconds to work the first 10 yards.
What can teams do? Lie. Hope the injuries don't get out. Make the injury seem more insignificant than it is. Make the injury seem more significant than it is (Patriots).
Would I lie as a coach in the NFL? Hell yes. I get paid to win not to help the league in their gambling endeavors. I would seriously consider that 20-50 games a year are directly impacted by injury reports. The NFL is full of close games. Knowing the opponent's weaknesses might be enough to sway a game 5-7 points.
