It is highly unlikely that a rat is going to harm your rabbit, it will take baby rabbits though, it is just after the food, if you were to take special care not to leave any food on the ground around the cage, and raise the cage up a couple of feet on round steel legs (not so easy to climb unless the legs are old and rusty) to deter the rats, you should be OK.
You could even buy a plastic rat bait station (about �15) specially designed to put rat poison in and keep everything but the rats out, and place it under the rabbit cage.