@chosen
We frankly don't know 100 percent for sure.
I have to tread somewhat carefully on this, but the working belief of many people inside and outside of the organization is that the injury was a combination of a severe blow to his sternum/ribs, and whiplash to his neck. Those reports have not deviated, though there is some uncertainty of the severity.
Concussions are obviously devastating injuries; but I think it can be somewhat easy to overlook other very significant injuries, that don't include a traumatic brain injury, that keep players out for longer periods of time.