Thank you for your query at DoctorSpring.com.
I understand your concern.
Horner's syndrome is one of the known but rare complications of ACDF. It is a surgical complication and may not be due to an incorrect sized or stiff cervical collar.
One can expect recovery within 4-6 months in case of a reversible Horner's syndrome.
I hope this was helpful.
Feel free to ask queries.