I would switch back and forth, put some cold compressor on the knee area up to the wing for 10 minutes, then the warm bath. Try to get the swelling and redness down and she/he should start feeling better. Only make bath time for about 10 maybe 15 minutes... Just a little each day and maybe add 1 minute each time you take her/him bathing until she/he starts to use it more.
It's going to take a while, plus make sure she gets her/him Vitamin b Complete. Just drop a pill in a quart of water. It's OK if the rest drinks it too. In fact it will help them in there future.. Make sure she's eating, pooping etc...