Suffering involves having preferences and not getting them – basically not getting what you want (which requires wanting something). A rock has no preferences about what happens, so it can't suffer. (A rock doesn't want anything, so there's no opportunity to be disappointed.)
The debate about animals should focus on whether or not they have minds involving preferences, rather than on mixing up physical pain nerves with mental suffering.
Pain nerves could easily be included in a robot that (that is, we could make a robot with sensors to detect damage and send information about the damage to the software running the robot). Despite the word "pain" in the name, it has nothing more to do with suffering that sensors to detect pressure or light or sound.
What people usually do is take attributes of animals that we see in today's video game enemies and then say "see! animals learn! animals are just like people!"
An indication that animals do not have preferences is their inability to create new knowledge such as philosophies. This shows they are not universal knowledge creators like people. (Universal knowledge creator = able to create any knowledge that any knowledge creator can create. Think generic knowledge creator rather than specialized within some limits.)
So an animal would have to either be a special case knowledge creator that somehow creates preferences but not philosophies, or else it can't suffer. There are no reasonable proposals for how this would work. The underlying problem is the people debating this topic largely aren't familiar with the key concepts like preferences, universality, and knowledge creation.
If you want to understand universality better, and how knowledge is created, the best place to start is http://beginningofinfinity.com