Are you sure it's not splayed leg? Feel the leg, see if it's broken on a bone or if the joint is dislocated. I don't see how that can happen on a chick. Typically this is splayed leg where the leg never really set into the hip position. That happens at hatch time and those that frequently aid hatches have a larger number of chicks with the issue. As the leg doesn't want to stay under the bird the condition worsens. Best to remedy it ASAP. The sooner it's in place the faster it heals correctly. Occasionally it can't be fixed.
Shackle the legs. Use a band aid or what have you to tape the legs in shackle fashion so they have width to take short steps but not enough give for the leg to splay out. That's it. Devise any method to do this with least hindrance to the bird. I cut down band aids t be narrow. In severe cases some go as far as putting the chick in a tapered cup to force legs under it and basically hand feed and water it for few days. You will have to make sure your weak bird drinks enough water today. Likely why it's weak- getting dehydrated due to inability to get to water and feed. If you've a vitamin and electrolyte mix then use in water if not sugar water to give it energy. The chick needs an energy boost to get it to eat and that in turn will get it back to vigor. Worse case use an eye dropper and give it a few drops of sugar water then shackle it.