While it is possible that a baby could have innocently been left alone and unclothed with this dog and it would have of it's own volition begun to attack this baby within a short time in such a way as to leave serious injuries requiring reconstructive surgery.
What is more likely is that some human abuser did the deed and when the baby's injuries were too severe to ignore, fabricated the dog story to explain said suspicous injuries to a medical professional.
Sounds like a a good assessment to me.