Actually, there is a lot you can do to try to save it. It may work, or it may not, but you will at least know that you've done all you can. Try this:
1) Keep it very warm, about 95 degrees. Isolate it from the others if you think it may have a communicable disease or if its so weak the others are beating it up
2) Give it an electrolyte solution like Pedialyte. You can make a substitute in a pinch by disolving a tablespoon of sugar or honey, an 8th of a teaspoon of salt (sea salt is good, but table salt is ok) and a tiny pinch of epsom salts in a quart of warm water.
3) Give it antibiotics. Tetracycline and either penicilin or a cephalosporin.
4) Force feed if necessary. You can make a slurry of boiled egg and chick starter with enough water to process in a blender or food processor - should be thin enough to go through the barrel of a syringe, but thick enough not to run -- consistency of toothpaste or pudding is about right.
I've saved a number of weak birds with this approach, and never regretted doing it. They can recover and grow up to be perfectly normal and healthy.