So zoro has a scar along his right eye and he keeps it closed. Maybe he trained his eye too to the point he has faster reactionspeed or sharper sight or whatever. Afterall he trained with Mihawk HawkEYES. But on the otherhand it would be a ripoff of Kakashis sharingan so i rather hope mihawk sliced it out in some intense training and Zoro couldn't give a damn about it and put a sword to his throat.