Seriously? The law has nothing to do with liberty mutual deciding it's not work related . Dr gives them the findings and they decide. If you don't report it with exact date and time and location of when you felt it , your screwed. Seen it happen many times to stupid drivers .
Not true.
1. Liberty Mutual can, and does, often base their findings on the "manager's report" which may conflict with the doctor's opinion.
2. Exact time and date on repetitive motion injuries are not always needed, or even possible. Any competent workman's comp attorney knows how to deal with claims being denied just because an exact time of injury is not established. Just because Liberty Mutual initially denies coverage does not mean they do not have to eventually pay. I know several UPS employees who were initially denied comp but later had it reversed.
The OP should seek out his own doctor if he is currently using the UPS doctor. He should also contact his State's worker's comp hotline and ask for advice. Also ask for references for a good worker's comp attorney from others who have had success fighting this fight, the hall is a good place to start.
Ask for any monetary penalty your State laws provide for...for unfairly denying your claim. Do not delay, there are usually deadlines.