This paper presents an online index for adaptive protection of microgrids concerning the low inertia concept. A three-step approach is developed to consider two offline and online environments, and the proposed scheme is designed. In Step-1, a comprehensive study of the microgrid dynamical model is investigated through two different distributed energy resources (DERs), including generator-based and inverter-based resources. In this step, based on the low-inertia concept, a dynamical model of an adaptive protection scheme is proposed through a set of non-model-based indexes. In Step-2, considering online evaluations of produced indexes, two different equations are provided in the form activation point and triggering point which the corresponding adaptive protective zones are provided. In this step, considering different operational and topological points of microgrid, the protective abilities of proposed indexes are validated and reorganized. Finally Step-3 concentrates on the ability of the proposed defensive scheme in real-time operating mode. In this step, considering different fault scenarios, the accuracy estimation and fast detection of the developed protection scheme are investigated, and the microgrid dynamic security is evaluated. Results present the effectuality of the proposed scheme to fast and secure detection of unstable scenarios using adaptive non-model-based signals concerning different system structures.