I think someone like Luke Staley is injury prone. Mathews might be injury prone. Guys who have things happen easily. Guys who are always pestered by injury because their bodies just can't handle it all.
Freak accidents don't make you injury prone. Taking a helmet to the knee doesn't make you injury prone because that knee is going out regardless of who you are if you get hit like that. Says nothing about how prone you are to being injured. It's just an isolated event. And a even a few isolated events all unrelated, don't really make you "injury prone" if they're just freak injuries that would happen to anyone.