Mint is REALLY hardy. As long as I can cut some pieces that have some smaller root hairs, those are good to replant. You almost can't kill mint. If nothing else, just take a sharp soil knife and cut out a section of the root-ball and replant it back into a large pot with some potting mix.
If that isn't possible, cut some nice healthy cuttings from the part of your mint that is growing. Strip off a few of the bottom leaves and plunk the stems into some water. You will get roots growing on those cutting within a week, and those can be planted into your container.
