It is oviously that he got some power up from his closed eye, I have a very good reason why I'm saying so, if he has lost his left eye, all his crewmate would have asked him of it, I have a feeling they already did, but we will find out the time he reveal what is really behind his closed eye.....so far its safe to think he dint lose his eye based on the interaction the crew is having, first thing first Chopper is a good docter he oviouly got worried when he saw Zoro with one eye and asked but we are all kept in the dark until he meet someone worthy or a very proficient swordsman, I have got a feeling that Both Zoro and Luffy received powers to take on admirals and yonkou, like the way they dint even show fear meeting up with Fujitora like they did wen meeting Admirals Two years ago!