Am I able to stand duty while assigned light duty for my leg?

I've recently gotten acl surgery. I'm on my last 30 days of light duty and immediately going on limited duty afterwards. My chit limits everything involving my leg aside from short walks, to include all forms of Physical Training not cleared by my doctor. I've been assigned duty later this month at a 4-story building with a broken elevator and when I addressed my staff, I was told that if I'm not assigned limited duty, I have to stand watch. I obviously don't want to stand duty, but aside from that, I'm pretty positive I'm not fit for it. Are there any orders that dictate this circumstance? Or do I have to get my doctor to write "unfit for duty/standing watch" somewhere very clearly?