To me this transfer doesn't make very much sense. We don't want to rely on Vermaelen, I get that, but Lenglet isn't very young, he hasn't showed top-quality and he isn't a once-in-a-generation barca-style player.
My guess is that this an ad hoc buy, because Arthur is suddenly coming in the summer (while previously thought to be impossible) and we therefore have 3 non-EU spots occupied.
We don't want to bet on Vermaelen (and we shouldn't), therefore we need another meh-quality CB, because in the end we hope that Mina will explode. I somewhat hope that Paulinho gets sold because we can't refuse a big offer, and we buy a replacement for him instead, while keeping Mina. Then Vermaelen can be replaced by a talented youngster like De Ligt this summer or next year.