I think it is the second time I encounter this problem. I mess around with my operating systems a lot. After backup and restore using rsync -avz, network-manager won't start on boot. I've search anywhere and none have the solution for me. So after getting a hint from Ubuntu bug "Waiting for network configuration" on every boot, I found out there is a log for upstart. So ...
$ sudo grep network /var/log/upstart/*.log
and found an interesting entry:
/var/log/upstart/network-interface-lo.log:/etc/network/if-up.d/upstart: 9: /etc/network/if-up.d/upstart: init_is_upstart: not found
and google the error:
init_is_upstart warnings if upstart installed but not running
note this entry from the bug report:
The only thing missing from backup restore using rsync -avz. Don't know why, I am just glad the restore works ...