I'm not confident he stays healthy, but I also don't think injury prone relative to other players a the position is a real thing in most cases. Finely-tuned large fast humans colliding with other finely-tuned large fast humans over and over makes every player injury prone. The fact a guy like Penny has suffered 5 or so injuries throughout the course of the thousands of collisions or the hours of tuning seems more likely to be bad luck to this point than to indicate he's just more susceptible to being injured than other players.
So yeah, if it were only the injuries suppressing his ADP, I'd say we can't predict that as well as we think we can and I'd happily take the injury discount. My bigger concern is that he's a 2 down back on a bad offense.
I used to subscribe to that theory as well, but some players, either due to their style of play, their lack of specific training or conditioning, rehab commitment, ability or willingness to play through injury, or just plain physical durability, succumb to injuries at a different rate than other players. Penny, for whatever reason, has been one of those players who has missed an unusually large amount of playing time due to injuries. Perhaps it truly is just a string of really bad luck, as you suggest, but that seems less likely to me than that he has been more susceptible to injury than the typical RB. Ignore Penny's history of injuries at your own peril. That said, I think we are mostly on the same page with Penny's value.
When he has been on the field, Penny has displayed signs of his first-round talent. I am not completely out on him for fantasy purposes. However, you have to build into his price not only his injury history, but an offense on the decline and the eventual presence of another very talented back.
Even if Penny picks up where he left off last season, once Walker returns, it seems likely the Seahawks will work Walker into a sizable share of the workload. For an offense that is likely to struggle to get into the redzone as often as in previous years, a RBBC may doom the values of both Penny and Walker.
If you have Penny, you are probably starting him Week 1. If you are not starting him now, when will you start him? He enters the game healthy and without a committee situation for this week, at least. Start him, and then look for a trade partner if he performs well.