Just a wild guess, but could it have gone into a skimmer or rubbed against concrete block. This looks more like trauma, and it is now a wait and see if the fish develops an infection. Keep an eye out for any reddening of the area, indicating infection. If it does happen, I would prefer that you have some Tricide Neo on hand, though some have had good results using a triple antibiotic ointment for treatment. Either way, you will need to have some clove oil or MS-222 to sedate the koi for treatment.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard