By using the diagram technique method, we introduce the collective variables in the theory of copolymer melts without appealing to the random phase approximation. The main result of this work is that the fluctuations in the collective structure factor directly couple to the individual structure factors of the block copolymer chain entering the Leibler expression of the collective structure factor. The expansion parameter, which controls the fluctuations, is found to be proportional to the Flory-Huggins chi parameter. We apply our method to compute the collective structure factor of a diblock copolymer melt above the microphase separation (MST) in a self-consistent way. Below the MST, we predict that the individual quantities of the copolymer melt (gyration radius, R(g); mean-square end-to-end distance of the block copolymer, R(2)) become explicitly dependent on the order parameter and compute this dependence for R(2) up to the lowest order.