The incident you describe would have probably been a tough call under any circumstances.
Basic first aid training should provide the guidelines you are looking for. There are always some things that you call an ambulance for--unconsciousness, profuse bleeding, serious fractures etc. Then there are the things you transport to the ER--anything needing stitched up, minor fractures, minor eye injuries, etc.
Some things can almost always wait to go to a clinic later--strains, sprains, back injuries, etc. In a lot of these cases a little rest and sincere expressed concern can go a long way toward avoiding that trip to the ER or clinic.
You want to avoid the ER unless it is needed. Once the employee goes there, your control of the outcome is diminished significantly.
Many of my peers are much more conservative about this. And I agree that you need to ensure proper care. But my experience with ERs is that once they go there, it will be recordable regardless of what it is.