Lots of people use the term "injury prone" so casually nowadays that it has lost its meaning. I was actually thinking to myself today that some people will likely start calling McNabb injury prone after hearing about his ACL injury.
Anyways, people can call players injury prone all they want, and I'm not going to make a judgement on it without researching a bit myself. But concerning McNabb, it seems more like age is catching up to him. He is 30 years old after all.