An assembly includes a cement retainer, a unidirectional flow valve, and an upper stinger. The cement retainer is adapted to be mounted in the casing string. The cement retainer has a retainer aperture extending longitudinally therethrough from a retainer top end to a retainer bottom end. The unidirectional flow valve is configured to selectively control flow of fluid through the retainer aperture and is housed within the cement retainer. The upper stinger is configured to be removably retained within the retainer aperture. The upper stinger has an upper stinger aperture extending longitudinally therethrough from an upper stinger top end to an upper stinger bottom end. The upper stinger comprises an opening that provides a fluid flow path from an interior of the casing string to the upper stinger aperture when the upper stinger is removed from the cement retainer.