Claudius I don't want to sell Havertz, I like Havertz. As you point out he has been productive, he is very intelligent and versatile, his athleticism is brilliant and he seems to be well liked by all.
However if for example we had Gyökeres and he was working out up front, then there may be a strong argument to sell Havertz next summer when he will have 2 years left.
We could get a hefty fee in order to recruit a £50–100m replacement with greater "specificity". It would depend on what Havertz might or might not be giving us by that stage. How useful will he be as an impact sub, suspension or injury stand-in, or rotation player?
Claudius The obvious issue on the team is how the left side works.
Still say most of our problem on the left is not Martinelli but our tactical outlook and its heavy emphasis on the right.
I have very real doubts the wide players on our list will either relish having the thankless job of running to give us all that width for 70 minutes twice a week, or flourish when required to have the beating of a couple of Premier League quality defenders to produce anything.