We have never been good defensively. Our record was good (the best in the league). But a large portion of that often was our ability to hold on to the ball for long periods, in most games. Against teams who deny us that possession, we become frail. We should be a better early pressure team. If we are not, we will always be frail when counter attacked.
It is not any particular player or players: Kompany is way better than any other of our defenders, yet we are often still frail defensively, even when he is present. Barry, De Jong nor anyone else could do much to reduce that (sure Barry conceeded some fouls to stop the tide and De Jong put in a few crucnhing tackles here and there). But whether our defense was frail or not, was often a function of our posession. We are great defensively when we control possession for long period, deny the opposition the ball often, and defend then score. These are our best defensive qualities.
Contrary to popular belief, we are a team who for the most part needs to spot defend. I.e. Our defenders (especially in the league) will spend a greater portion of their time supporting possession, and initiating attacks, rather than defending.
This is why I have for years cried for a ball playing CD: It is why I wanted Luiz, wanted Vertonghen, still want Filipo Luis, and wanted Boateng to stay. Sure being a great defender is good, but for a team that will be pinning the opposition back as its primary form of defending, a good defender with great possession skills is way more important. Luckily for us, Kompany possesses both. Nastasic is good enough with both, and togeher we may keep up both possession (and by default a strong defense.)
We want a stronger defense? Possess the ball better, attack with better ball possession and chance creation, and we will!