Crit healing is a problem for me. It's quite possible for a mangled limb or a concussion to heal over night. Realistically, wounds like this take weeks, if not months to heal. I'm not looking for the game to mirror real life, but I do want less cinema and more grit.
How about:
A crit *cannot* heal until successfully tended by a character with Medicine or First Aid. The severity is the number of that is added to the medical check. Each crit must be tended separately.
Resilience checks to heal crits can only be made once per week by the suffering character.
The severity represents a number of wounds that will not heal until the crit is healed.