you do not need an mm to implement HA, please see page 74 of the AOS 8 fundamentals:
It should be noted that clustering is not supported by Standalone controllers. If Standalone controllers must be used then their primary redundancy mechanism is HA.
and then on page 186 you can find more details, caveats and the like
Standalone MC and Standby Standalone Topology
This ArubaOS 8 design consists of a standalone MC backed up by another standalone MC. As in the Master and Standby Master ArubaOS 6 design, VRRP is used between the two standalone controllers in an active-standby configuration. Similarly, High Availability (AP Fast Failover) is configured between the controllers so that APs terminate their tunnels on the active standalone controller in addition to setting up a standby tunnel to the standby standalone controller.
Of course there are a bunch of functionality impacts etc. that go with not having an MM, but that's a separate topic and I think already pre-determined for your environment.
hth.