This paper deals with LTI interconnected systems whose subsystems have coupled dynamics. The objective is to decentralize a given centralized controller satisfying some prescribed design specifications. More precisely, a parameterized decentralized controller is to be designed such that the state and the input of the system under the obtained decentralized controller can become arbitrarily close to those of the system under the given centralized controller, by tuning its free parameters. To this end, a two-level decentralized controller is designed, where the top level captures the dynamics of the centralized control system. The notion of structural initial value observability is introduced to design the bottom level of the decentralized controller. This method can decentralize every generic centralized controller, provided the interconnected system satisfies very mild conditions.