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Weight loss on a dog should be gradual. Since the dog needs rest the next couple of days, invest in a weight loss dog food and start adding that with what the dog normally eats now so the dog gets used to is slowly.
Start with 1/4 new food t 3/4 old and gradually increase then amount of new food while decreasing the old.
Once the dog is able to get up and about, take it for a walk each day , play fetch games etc. as long as weather permits.