I've been watching Liverpool since I was 7, in the year 2000. I haven't seen a better LB in 18 years of watching. Riise was good for a number of years, but not this good. Traore was...entertaining. Aurelio was great, but didn't have this kind of energy. Dossena was average. Paul Konchesky was a symbol of our decline. Aly Cissokho was poor, except for 2-3 amazing performances that included a brilliant assist for Suarez's extraordinary header from outside the area. I think Victor Moses played there for us as well--I don't remember. Moreno was poor until last season, when he became decent. Milner was decent, but clearly not a left back since he didn't like using his left foot!
And finally after all that, we have this monster full back. He may not quite have the elite overall technical ability of Marcelo, Dani Alves and other top full backs of recent years, but he has good enough overall technique and literally every other ingredient of a top full back: pace, stamina, defensive positioning, one-on-one defending, decent passing, decent ball control and ball-carrying, crossing, final ball. Additionally, he has the mentality of a winner.
But, seriously, why did it take 20 years?