To be clear, othopedics is not my area of expertise,
edit - ortho specialty chimed in. Sounds like it's responsible for much less than I understood - see the next comment in the chain.
so grain of salt, but to my understanding, it's basically the radius bone, but of the leg. Aim your toes forward in the normal anatomical position; now rotate your entire foot to the left or right. That's your fibula working. Without it, very limited rotation; but otherwise you can still walk and function mostly normally, just a bit impaired.
I have no idea how 'chunk missing in the middle' plays into that. Guessing you'd still have some rotation, but without a secure anchor point, it wouldn't be great.