It is officially supported on two different scenarios:
1. Support for minimal customer configurations. Some customer sites use a simple VMware topology, comprising a single ESX host, with one or more virtual machines residing on that ESX host. These sites typically do not implement a vCenter management layer. However, the virtual machines residing on a standalone ESX host still must be backed up in order to protect against data loss. Adding the standalone ESX host as an Avamar vCenter client enables those virtual machines to be backed up with image backup, rather than guest backup.
2. Virtual vCenter disaster recovery. Adding an ESX host as an Avamar vCenter client can be useful when virtual machines residing on a particular ESX host must be restored, but the vCenter is not operational. This is often the case when a virtual vCenter must be recovered from Avamar backups. Adding the standalone ESX host as an Avamar vCenter client enables the vCenter management infrastructure virtual machines to be restored so that the vCenter can be restarted.
Yours is the second one, so check VMware guide “Protecting ESX Hosts” for limitations and further information.