The following answer is taken from Migrant Workers and Workers' Compensation: What You Should Know produced by Industrial Accident Victims' Group of Ontario.
It is best for your case if you and your employer tell the Workplace Safety & Insurance Board (WSIB) about the injury right away. You must tell the WSIB about your injury within 6 months, or you could be unable to get benefits. In most cases your employer is required to report any injury that requires health care or results in you not being able to earn your full wages. The resource below sets out the steps you should take if you experience a work-related injury or illness.