The paper studies the problem of multiplexing downlink enhanced Mobile BroadBand (eMBB) and Ultra-Reliable Low-Latency Communications (URLLC) traffic in 5G Massive Multi-User Multiple Input Multiple Output (MU-MIMO) systems. For eMBB users, channel resources are al-located in advance. When a URLLC packet arrives, the base station shall urgently reallocate resources in such a way that URLLC latency and reliability requirements are satisfied while avoiding degradation of eMBB throughput. Using extensive sim-ulations with NS-3, the paper evaluates various approaches: (i) to select transmission parameters for URLLC user and (ii) to reallocate power between eMBB and URLLC users. Based on the results, the paper provides recommendations for designing an efficient MU-MIMO scheduler to solve the aforementioned problem.