You're missing the point here. I'm not doubting Kalas's quality but he's still relatively inexperience and may well need time to adjust. Just because others from the weaker leagues have succeeded, doesn't mean everyone would, at least not immediately. Still won't know how will he fare and mind you, it's different pressure altogether playing with us challenging for major honours and players like the one you mentioned playing at clubs with lower ambition & different objectives. Moreover as I have already pointed out, if we sell Luiz, we would then have only 3 experience but average CBs in Terry, Cahill and Ivanovic. Partnering one of them with Kalas doesn't exactly fill me with confidence and also for the reason CHOULO mentioned.