First, I am not a doctor, not do I have any more information about Votto's injury than anyone else... but after reading the thread on Votto in ORG I wanted to add from my personal experince on neck inuries and hopefully dispel some misinformation.
No, they do NOT all go away within two days.
No, they are NOT caused only by car accidents and anxiety.
Yes, even a minor stiff neck can be physically debilitating.
Most of the back muscles, some of the strongest in the body by the way, attach to the neck. A post by Vottomatic mentioned that he pulled back hard from a pitch the night before he went out and then winced. From personal experience in the field I can tell you that this is an entirely plausible explantion that Joey would sit out for a while. I am not saying that is the definite answer... just that this is plausible. I have had the same thing happen during field exercises.
The dilema with a neck injury of this type is that if Joey did flinch back hard and the back muscles pulled unevenly on the neck, the resultant soft tissue injury makes him especially prone to do the exact same thing again the next time he puts a lot of strain on that area of the body unless he gives it time to heal.
That's why you dont just take an aspirin and goto work... you need to take a few days without physical strain to allow the inflammation and trauma to heal... then you can be as good as new. Your neck actually heals very nicely if you give it the chance.
Then again, if you are an idiot, like I am, you pop some ibuprophen, strap your kevlar back one and get back on the job. Then the injuries start to become more and more frequent until you end up getting major neck surgery (2 actually) and now over half my neck is fused together. I can't even type something as short as this without the neck stiffening up.
If Joey looks sad sitting on the bench, maybe he just hates not playing... in that case, kudos to the Reds medical staff from protecting him from my brand of idiocy.