If an animal has a minor injury that impedes but not prevents performance, healing would work well without assistance.

But we all know someone who's had a total snap fracture. Animals can do this too, right? If given enough time.

The reason the above answer addresses your concern is that a set major bone fracture is similar to a minor fracture in many ways, so if you can already heal minor fractures then you can heal set major ones in the same way

Healing broken bones is not something we evolved to do as groups. Until about a hundred or so years ago a broken leg was considered a fatal injury

That would imply doctors couldn't set a broken bone in the Revolutionary War? The Civil War? Of course they could! They could even do amputations.

A broken leg could be fatal if the broken bone shard cut the artery. That can happen even today.

But plenty of people in premodern times survived leg fractures, though often at a cost of a lifelong limp.

It's not about the initial injury. How are you going to find food when you can't move properly? How are you going to avoid predators? If you are alone and not in a group that can help you with these things until you heal it's pretty much a death sentence.