Do not wait until the employer asks you. Be pro-active, show that you are a responsible team member, and call him!

Call him TODAY and say that you are in pain and cannot use your hand. Inform him that you do not know when you will be back at work.

Tell him today that you will not be at work tomorrow. That gives him a chance to call up someone else to work in your stead, at least for tomorrow, or to re-distribute the tasks across other staff members.
Tell your boss, TODAY, and that you will contact your doctor as soon as his/her practice is open in the morning, and that you will phone him again after your appoinment.