In this paper, a new and novel approach called (Unmanned Areal Vehicle) UAV-Direct is proposed, where the UAV helps in optimizing the bandwidth and power allocations of Device-to-Device (D2D) communication. In addition to that, the UAV can play the role of a relay if needed to maintain the communication links between the devices that are out of communication ranges of each other. We design a UAV-Direct protocol to manage the resource allocations to improve the system throughput. We formulate an optimization problem that maximizes the minimum device throughput while satisfying resource allocation constraints. Since the formulated optimization problem is non-convex, we propose to solve this problem in two steps. In the first step, a Taylor series successive convex approximation solution is proposed to optimize the resource allocations. Then, we propose an efficient algorithm based on a recursive shrink-and-realign process to optimize the UAV trajectory.