The Rangers policy is to not specify injuries. In the past, at the very least we were told whether it is 'upper body' or 'lower body'.
The Girardi injury last week was the first time I remember the team, and players not even discussing what it could be. Upper body? Lower body? Nothing.
PCS (Post-Concussion Syndrome) does not always show up right away. It can take days, even weeks. If that Lucic hit did cause a concussion, he could have started feeling symptoms a few days later. It might have actually made him feel physically sick (flu like symptoms). The warm-up, and subsequent Washington game could have been him wanting to tough it out, and do whats best for the team. Rick Nash is a tough dude, and a true hockey player. The guy is a warrior, he wants to do what is best for the team. It might not be the most intelligent move, but thats how competitive this guy is, which is a huge reason I love him as a player.
This type of injury isn't an exact science. It's not always easy to initially diagnose, and since that hit wasn't the type that Powe suffered, the medical staff could have not even noticed that it could have done damage. At that point, it comes down to the individual player coming up to a trainer or doctor and telling them they aren't feeling well.
It's all speculation at this point.
It could be his groin, it could be a concussion, it could be anything. Let's hope he's back ASAP.