Your whole premise is invalid.
If you say you were injured on the job and your employer says your weren't, it's not up to your employer (or his insurance company) to present any evidence at all if they don't want to.
It's up to you to prove you were injured on the job. It's up to you to investigate and obtain witnesses who will go to court and testify that you were injured on the job.
If you think somebody else is going to do that for you, you are mistaken.
Now tell us what happened to you instead of making this seem like a vague hypothetical question.