there is a chance that they may reject it and I would have to extend my ORD

It is (in my opinion) unlikely, but possible.

The main crux here is whether the MC is deemed to be valid or not. I am not too sure on the medical board/review process, but if it is deemed not valid, then it is a big fucking deal. In my opinion, it is highly unlikely for it to be deemed not valid since it's from public hospital after surgery. Usually, MCs are only invalidated due to malingering etc. However, I am not the medical board and I am not the MO.

Suppose it is deemed as invalid, then the period of absence would be considered as AWOL, because you are absent without valid reason, since the MC is invalid. However, it is not a simple extension - processes have to be gone through and depending on the duration, a court martial may need to be conducted. Only when this happens, you should probably go back and ask the surgeon for a memo, but I think it's a bit premature to do so now.