I disagree, he should be 3rd choice after VVD and Gomez. Both of the others have pace so they will compliment Nat. I would sell Matip and keep Kabak as 4th defender.
I like what he's done but there's no way he's better than Matip. If Joel can't stay fit, then yes, we need to find a replacement, but when he's fit, Joel is better than Gomez and better than a lot of starting CBs in the world. Phillips is a good 4th choice or maybe 3rd in a pinch, but if we wish to be winning regularly, we can't have him playing regularly.
Take nothing away from him, he's done a good job and done what's been asked in a tough situation, but Jürgen needs ball playing CBs for his system.
I'd rate Nat about the same as a Martin Kelly type. He'd probably do better to go to a smaller club and get regular gametime once we get our starters back.