Does onion Juice hurt the transplant area if you use it 2 months post-op?

Onion just has nothing to do with hair transplants.  You can drink it do what you may want to.  You may want to ask your doctor if you are concerned.

I doubt onion juice would harm your scalp or grafts at this point. But I also doubt it would do anything beneficial, either. But regardless always check with your doctor before using a home remedy on your head following a transplant.

Did your hair restoration surgeon recommend this post op? You should be following your clinics post op care program and instructions to the letter. Going off on your own and putting a bunch of stuff on your head after your procedure that you read about online could effect your result in a negative way.