I am always in favor of having a good backup goalkeeper. There should be an option that will not cause any problems with being a backup and will give confidence as a backup performance. Karius is not good at this. He has questionable performances and I'm not sure he can give the team confidence in Pope's absence.
It's complicated too actually as whenever you have a good backup goalkeeper, manager always have tough time dealing with options then. Take the situation at Arsenal for instance but you have to get a strong goalkeeper when main goalkeeper gets injured for a long term. Situation at Newcastle is same and I think best thing to do right now is to sign a goalkeeper on loan.