He's still massively underrated by the general footballing community. He makes defending look so easy, gets through pretty much every game looking like he just came back from a nice relaxing walk in the park even if its his 3rd full game in a week. There is not a single defender in the history of football who could do what he does at that level so consistently and effortlessly. At his peak, he was essentially a cheat code defender who could do everything and had the pace to go toe to toe with the fastest forwards on the planet. At 32, he has lost a yard of pace, where, he is still fast, just not quite so domineering, but he has adjusted his game slightly and continues to absolutely dominate his area of the pitch. That header for Kelleher was class, but there were so many during pretty much every game where he just effortlessly wins his duel and plays it first time to someone else. No fuss, just making incredibly difficult things look simple 25 times a game. Its rare to see him having to make 2-3 sliding tackles in a season, let alone each game.
I've watched Baresi and Nesta and Chiellini and Rio and Terry and loads of center backs tagged legendary and none of them were/are anywhere near as good as him. Its just nostalgia and bias that makes you remember them differently.
I can confidently say that he is the best to play in the last 30-35 years at the very least. Before that, I just wouldn't know.
