De Ligt could become the next best defender in the world or could be your next Nastasic/Mangala etc. We have enough young CBs ourselves, problem is our experienced CBs in Rojo and Smalling are gonna be both 29 this season and neither have been able to put out a consistent run of good performances. Smalling has done well overall this season but he always has a mistake or two in him.
Alderweireld is not only a top defender himself but most importantly he can organize a backline (which is where we are sorely lacking right now considering how many soft goals we've let in). Just take a look at this and you'll understand perhaps why Jose wants him and not the next infant.
He has 3 informs on FIFA this year, perhaps that's why