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I am so sorry your cat was hurt. A cats appetite is the best barometer of his health so as long as he is eating, he is not in a lot of pain. His rib may just be bruised.
If it was broken he would have difficuty breathing so as long as you do not see that just keep him as quiet as possible to he is seen tomorrow.
If he will let you apply a warm compress otherwise just let him be.
I hope he will be fine so please let me know how he is doing.
Sincerest best wishes,