De Ligt is already imo playing at a higher level than Alderweireld ever reached at Ajax. Verotonghen had a season at 24 that is probably the best centreback season in the Dutch league in the 21st century, alongside Alex at PSV and then followed by Sanchez last season. De Ligt is pretty close to that sort of performance and he's only 18. He's incredibly consistent. He's a leader. clever. defends aggressively but intelligently (think the aggression on the ball of Vertonghen but with the more clever timing of Alderweireld). Has a crosspass and a throughball as well. Only worry is still his mobility. If he gets any bigger with age he really could get in trouble. Fortunately he himself realises that and has been following individual training for over a year now to work on exactly that.
Also despite my hesitations about him as a coach, I'm happy he gets to work under Koeman. That's a player who, footballing, I've always likened him to since his youth days. He's two footed, can dribble, shoot, and has a brilliant eye for a pass. Koeman is the perfect person to give him that additional guidance in that (and apparently his instructions for De Ligt for last night were about his attacking input). It's so rare to have such a good footballing centreback who is at such a young age also already equally good defensively and physically. Usually there's one or two of those that needs to catch up. not the case with de ligt.