The emerging unmanned aerial vehicles (UAVs) are playing an important role to assist cellular networks and provide ubiquitous coverage for cellular networks. UAVs can eventually increase capacity and facilitate line-of-sight connections. In this paper, we develop an analytical framework to analyze coverage probability for UAVs-assisted small base stations (SBSs) with clustered users, taking into consideration mm-Wave and directional beamforming. Moreover, the contributions mainly include the consideration of multiple-input-multiple-output (MIMO) transmission. Additionally, the clustered user's locations are modeled as Matern cluster process (MCP). Using tools from stochastic geometry, we derive a general expression for coverage probability according to the signal-to-interference-plus-noise ratio, by considering interference between UAVs and SBSs. The analytical results are validated using Monte-Carlo simulations in which the transmit power of SBS and UAV are considered to be 30 and 24 dBm respectively along with a carrier frequency of 28 GHz. The association probability is calculated as a function of cluster size and UAV density with different values of UAV height. Similarly, the coverage probability is computed as a function of SINR. It is observed from the results that the association probability depends on the altitude of the deployed UAVs and the typical height of a UAV should be between 40-120 m in order to achieve maximum association with a user. Further, it is observed that an upsurge in the density of UAVs and cluster size also affects the association criterion.