So a piece of the black plastic underskirt came loose on the freeway and before I could find a safe place to stop, it fell off. As it flew into the car behind me it looked a lot bigger than the space it left behind that I can see underneath. Or is something else missing from this picture? 1. Either way, how important/urgent is it to replace? 2. What's the part number or description?
I would replace it: It's number 9 here: Fender & Components for 2014 Toyota Prius | Toyota Parts Currently out of stock there, but you can search for the part number: 53876-47070 There's a plethora of fasteners, check: some of them might have stayed with the car, and you can also compare to the other side.
Did you catch my typo, that it is in fact part no: 53875-47030 I gather you did, with your question. Good catch.
