a) he doesn't need to tell US anything, and by being completely honest about things like this he is possibly putting himself at a disadvantage (for example "Hello Rival GM, I don't really NEED your third baseman, but if you want to part with him I'll give you less than I would be forced to if I was in a particular bind...") it's possible he doesn't, in fact, know. He's not (to my knowledge) a doctor, so he's relying on his staff, and the word of the players.
Frankly, I could care less if "lies" to the fan base about the severity of injuries.