I think the injury thing is probably a safety net to his future that will allow him to go balls to the wall on every play knowing that if he does get injured (career ending?) that he will at least be able to take care of things. That's the way I read it anyway.
The injury thing is the same thing that Brady got in his contract. If Percy is cut for performance he doesn't get that portion of it guaranteed. If he gets cut for injury he gets that portion. What I don't know is how that works from a does the $11 million get spread on the life of the contract or is that last years salary so he is owed that in full etc.