Christensen should replace Luiz when Luiz declines. Christensen has been playing in the centre of a back three at BM. In my opinion we recalled him one season too early. This time next year, Luiz is going to be 31 and he will start showing signs of age. That's when he should be phased out and Christensen should be integrated.
Instead, if we buy VVD, we will keep Christensen on the bench for a season and sell him next year for £30m. He will then go on to be the next Hummels and will be bought buy a rival PL club for £70m.