This explains a lot. First, it could easily have happened as the bird took its first flight and landed on concrete, instead of on dirt and grass. It was able to walk so we didn't notice it. Second, it explains why the bird appeared weak and was not jumping up and down and practice-flapping its wings. Third, if the bird is in pain, just like a person, it is not in the mood to eat and therefore will feed less. Lastly, this can fall into the category of "human-caused" interference, since we're the ones who laid the concrete down, and hawks never evolved to have to deal with concrete.
I have immobilized the joint in a splint, and we have an appointment with Dr. Boutette for a radiograph on Monday. Despite the injury, it is actually quite strong and fights back viciously whenever handled."