After upgrading from 15.10 to 17.04 (via 16.04), the NetworkManager refused to control the wired ethernet device, listing it in its menu as device not managed.
Turns out after some googling that the package’s configuration was changed in 16.10 to consider ALL device types as unmanaged, except those explicitly listed. Which currently contains only the wifi and wwan types.
Solution:
Create an additional config file in /etc/NetworkManager/conf.d/ overriding the one /usr/lib/NetworkManager/conf.d/ with the following content:
[keyfile]
unmanaged-devices=*,except:type:wifi,except:type:wwan,except:type:ethernet
Then simply restart the network-manager service and all should be fine again.
Additional information: