It looks to me like the rat might have pulled a scale off ? You can clean the wound with plain warm water to get a better look at it.

I think if you keep it clean and apply neosporin {the kind without pain reliever } it will most likely heal on it's own.

I've seen pictures here of some pretty drastic wounds that got better with proper care.

Someone with more experience will chime in with more info.

Gale