If you are under doctor's order stating that you are unable to return to work, you are unable to return to work based upon medical reasons. I would send a copy of the medical excuse to your employer stating that you are not able to return to work presently.I would also request that your treating physician state in the medical release an estimated return date for you to be provided to your employer. In a nutshell, your employer cannot realistically expect you to return to work if you are on documented medical leave.