Depending on the breakage (e.g. if your initrd got borked), you'll probably need the rescue disk anyway in order to have access to the necessary tools to activate RAID arrays, decrypt disks, activate LVM VGs, etc.

Even without, IIRC, Ubuntu (one of the few systems that actually gets root passwords right by defaulting to it being randomly-generated garbage that nobody knows) will prompt for the credentials of a sudo-capable user even for single-user mode.