Yes. It does need treatment. As with an injury to any living being, this may get infected.
You need to quarantine this fish immediately, and begin treatment. First of all, move this fish to a separate tank, such as a spare fish tank, or a tub. Use water from your original fish tank in this (do not use fresh or new tap water). Set up an air-stone in the tank. Take out some of this water in a small bowl, and add some rock salt to it. Rock salt should be added in the ratio of about 1 tea spoon per 10 gallons (of the water in the tub). Salt will prevent some infections. Add this salted water gradually, and if your fish shows signs of stress such as thrashing or gasping, stop adding salt and add some water to dilute. Be advised that Salt stings, and you koi might get stressed out a bit. Any discomfort for more than a minute is when you should stop adding salted water.
In case any secondary infections are observed, use Merck Methylene blue. You can add 5 ML for each 10 gallons.
Since adding salt and methylene blue to your original tank will cause unwanted stress to your other fish, and also since Methylene blue kills the beneficial bacteria in your filter, quarantining is a must.