Since he was already having half his face transplanted, I think they did it just because why not. They wanted to prove they could re-vascularize the retina at least. He was aware of the risks and it likely already came from the same donor anyway so it's not like it created more of a rejection risk than whatever musculature they already transplanted. If the retina didn't get successfully revascularized or it was some sort of very localized rejection, they'd just enucleate the eye and put in a prosthetic.