If a nerve, such as the lingual nerve, was bruised during wisdom tooth removal was bruised then the numbness/pain will resolve on its own. If the nerve was accidentally injected with lidocaine it may resolve on its own. If a nerve was actually severed, treatment is needed and the SOONER it is done the better the chances of recovery.
85% of these cases resolve spontaineously. That is why dentists feel comfortable telling people "not to worry" because 85% of the time they are right.
If it were me, I would question the dentist regarding whether or not he saw the nerve during removal. If they saw the nerve there is a mighty good chance it was damaged. I would seek a consult with an oral surgeon experienced in nerve repair sooner rather than later to find out if treatment is needed. If surgery is needed the best outcomes are for patients who receive treatment within six months of injury.
Do a search on "Lingual nerve damage" or "Burning tounge" for more information.