In infrastructure-based wireless networks, installing multiple antennas on fixed relay stations provides an effective and low-cost solution to increase network capacity and combat hostile fading channels. Previous work on multi-antenna relays either assumes that only one relay available in the network without benefiting from relay cooperation or all relays are used to assist the source transmission resulting extensive loss of spectral efficiency and high burden to the diversity combiner. For multi-relay cooperative networks, adaptive incremental relaying (IR) is considered in this paper as the cooperative strategy for multiple fixed relays performing uncoded decode-and-forward (DF). Unlike previous work on adaptive IR, relays employ multiple antennas to enhance the received signal quality. Then a small number out of all available relays are used instead of them. In determining whether the rest of relays are needed, two kinds of mechanisms are considered, namely decoding and threshold test that characterize different operational complexity at the destination. For both cases, we derive closed-form expressions of the average outage probability over Rayleigh fading channels. Numerical and simulation results are presented to demonstrate the effectiveness of adaptive IR in the context of multi-antenna relays.