If you want your work injury to be covered, the employer needs to know you were injured at work. This can be accomplished by telling someone right away-generally a supervisor is better than a coworker. This can also be accomplished by actual notice-the employer witnessed the incident.
The problem arises when you, say for example, "tweak" your back on a Friday and do not mention it to the employer. Then say, it gets worse and worse over the course of the day. You still don't tell anyone. You go home. Over the weekend, it becomes unbearable. You then tell your work on Monday that it happened at work. Immediately the employer's insurer becomes skeptical of your claims since you played in a softball tournament that weekend or worked in your garden. The next thing you know, the insurer hires a doctor (IME) to say your injury never took place at work in the first place.
You may find this story far fetched. It's not. I just fielded this call this week.
The advice is: give the employer notice immediately. It will help get your claim covered.
If you have issues or problems like this, or need advice about any workers' compensation scenario, talk to a professional.