Slice the pork almost in half, but do not cut all the way through. Butterfly the sides until the pork is open enough to stuff.
Mix torn bread, melted butter, poultry seasoning, onion, and celery in a bowl until bread is evenly coated with butter and seasoning; season with salt and pepper. Stuff the pork tenderloin with the bread mixture. Wrap the pork around the filling and secure closed with toothpicks. Wrap bacon slices around the stuffed tenderloin. Season top of tenderloin with black pepper.
Cook in the preheated oven until pork is cooked through, about 45 minutes. An instant-read thermometer inserted into the center of the stuffing should read at least 145 degrees F (63 degrees C).