network manager is not required to connect to the Internet.. Quite frankly you could uninstall network manager, ensure your /etc/network/interfaces file is correctly configured for your ethernet card and you should be just fine.
userer@it-lenny:~$ cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
iface lo inet loopback
# The primary network interface
iface eth0 inet dhcp
Personally I find the network-manager application to be annoying. Always telling you the cable is unplugged when you have a static IP address assigned, so usually i just remove it. if I must use a graphical network management application I much prefer wicd