You'd sell one of Lovren or Matip, not both. That'd be asinine.
Exactly. Considering we would be buying a CB because Gomez and Lovren are injury-prone, it wouldn't make any sense to ship both Lovren and Matip away to find ourselves with 3 CB instead of 4, and we'd still be carrying one that is injury prone in Gomez.
I agree with the argument about the need for a new CB if Gomez is indeed injury-prone (I can't judge that by myself). It doesn't make sense to say that our defense is great only when Gomez is fit and then watch him miss an insane amount of games especially when you consider that the backline needs regularity and stability in order to forge the best possible understanding between the players.
As to who we should buy, I don't have a specific opinion on that, but I think that top money should be spent in that area even if it means spending most of our budget on it and missing out on a top AM and a striker. There is no doubt in my mind that heading into a season with a problem in our defense is by far the worse problem of the two.
We've seen what a difference it makes to have a defense capable of dishing out clean sheets. You basically have to score a goal and most of the time the job is done. Compare that to last season when we regularly scored 2+ goals in our games and still lost an insane amount of points because we couldn't hold on to the lead. No question, secure the backline once and for all at all costs.