From what we have seen this season, Bellerin is not a good defender, likes to go up and needs Mustafi to cover his place.
Monreal is another year older and deteriorates like free fall. If he is our starting LB next season, he could be our weakest link of our defensive line (I won't say so ATM). Gibbs??? Don't kid me.
Mustafi, probably the best to control and distribute the ball among all defenders. A good CB no doubt . I don't have stats in front of me, but whenever I watched (about 10 times) him play, he likes to carry the ball up front (probably our DM/CM can't do this??). He is the opposite of Per, who likes to sit back and protect the other CB.
Kos, our best CB, got hotheaded once in a while, also like to bomb forward. So far I like the Per-Kos combination more because they cover each other weaknesses. Mustafi-Kos combo doesn't give me that kind of confidence.
That's why I believe an extra protection is needed.