When must I call an ambulance instead of just removing the player?

The Hockey Australia Concussion Policy (effective 1 January 2024) says that if no qualified medical professional is on site:

• Possible structural head injury: signs such as abnormal neurological signs, a persistent or worsening headache, increased drowsiness, persistent vomiting, increasing confusion or seizures usually persist or worsen over time. Do not attempt to treat or move the player. Call an ambulance immediately and wait for it.
• Possible neck or spine injury: suspect one if there is any loss of consciousness. Do not move the player, other than to support the airway. Do not remove protective equipment such as a helmet or face mask unless trained to do so. Call an ambulance and wait.

Respond with basic first aid principles: Danger, Response, Send for help, Airway, Breathing, CPR and Defibrillation. A removed player must also be monitored regularly for signs of deterioration.