A protective sleeve is provided for use with a low-profile boot to provide a resilient layer that prevents penetration of debris accumulated at the interface of the boot with the pastern of the horse, especially at the rear of the hoof. The form of the sleeve is tailored so that it conforms to the natural shape of a typical hoof and pastern without creases or other unnecessary accumulations of material that might provide discomfort to the horse. The lower portion of the sleeve is adapted to wrap around and cover the heel bulb, but not the rest of the hoof, in order to retain the ease with which the boot is put on the hoof of the horse. The lower portion of the sleeve is designed to wrap around the pastern and overlap for fastening with a loop-and-hook strap. Also, a retaining flap disposed upon the protective sleeve and located between the horse's hoof and the interior of its boot allows the sleeve to more securely stay in place around the animal's hoof and pastern during movement.