Heat the olive oil in a large pot or Dutch oven over medium-high heat. Season the pork with salt and pepper and cook, turning, until browned on all sides, about 7 minutes. Remove to a plate, reserving the drippings in the pot. Add the celery, onion and mushrooms to the pot. Cook, stirring occasionally, until the vegetables are lightly browned and tender, about 5 minutes. Add the wine, scraping up any browned bits, and cook until reduced, about 1 minute.
Add the chicken broth to the pot and bring to a boil. Reduce the heat to medium low and add the pork and rosemary. Cover and gently simmer until a thermometer inserted into the center of the pork registers 145 degrees F, 10 to 12 minutes. Remove the pork to a cutting board and let rest 5 minutes. Discard the rosemary. Add 1 tablespoon butter to the pot and stir until melted; season with salt and pepper. Stir in the parsley.
Meanwhile, bring a medium pot of salted water to a boil. Add the couscous and cook until tender, about 5 minutes. Drain and stir in the remaining 1 teaspoon butter; season with salt and pepper. Thickly slice the pork and serve with the couscous; top with the mushroom sauce.