The present invention provides systems and methods for protecting the lower main wellbore of whipstocks and completion deflectors from debris accumulation. A well system subassembly includes a deflector tool arranged within a casing string and defining a deflector surface and an inner bore extending longitudinally from the deflector surface and one or more seal stacks disposed about the inner bore of the deflector tool. A wellbore barrier device is disposed about the inner bore uphole from the one or more seal stacks and is expandable from an unswelled configuration to a swelled configuration. When the wellbore barrier device is in the swelled configuration, it protects the one or more seal stacks from debris generated from milling and/or drilling operations.