To enable uplink (UL) multi-user (MU) transmission in 802.11ax, the Access Point (AP) sends a Trigger Frame to schedule stations (STAs) to send their UL data, and uses the new MU Request to Send (RTS) mechanism to protect large MU transmissions from hidden nodes. This paper addresses the potential problem of having one or more solicited STAs with nonzero Network Allocation Vector (NAV). This would reduce the system efficiency as some allocated resources will not be exploited for the reserved uplink transmission period. Accordingly, this paper proposes a variant of the Trigger operation considering the employment of massive multiple input and multiple output (MIMO) on the AP side, designated as Massive MU RTS (MMU-RTS) mechanism. In the proposed mechanism, some backup STAs not included on the trigger list, but having zero NAV are permitted to respond to the AP together with the solicited STAs. In particular, two user selection algorithms are proposed for the MMU-RTS mechanism. The efficiency of the proposed method is demonstrated through extensive numerical simulations.