I think there is a "continuum" in what is an injury. There are some like broken knee or whatever which we would surely call an injury.
There are other injury, like Tommy and andi roddick have said, which are more soreness and tiredness, which they must withdraw for but they have to be called "injury" or they cannot easily withdraw saying "sorry I want to rest" (which they might give a fine for
Because of problems with my right shoulder I won´t play in Las Vegas.
After a short visit in Las Vegas I am at home in Bradenton right now. The doctor recommanded me not to play this week. My shoulder needs a short break. After all this problems I had in 2003 I´m very careful. But I should be ready for Indian Wells next week.
I think he is just being careful, and with his history of injury problems I can't blame him. I am not a fan, but he is a good player and he is doing well this year and so needs to keep strong and injury free for the important tournaments. Good Luck to him.
Supporting Djoković, Tipsarević, Soderling, Tursunov, Youzhny, Cilić and quite a few more