The documentation. 'Configure ArcGIS Pro licenses' with Portal for ArcGIS does not specifically mention anything about ArcGIS License Manager 10.5.1 or later being required to manage ArcGIS Pro 2.0 Named User licenses.
The Configure ArcGIS Pro licenses documentation (https://enterprise.arcgis.com/en/portal/latest/administer/windows/configure-arcgis-pro-licenses.htm) now includes a compatibility matrix that includes ArcGIS Pro, ArcGIS Enterprise, and License Manager versions.