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It is a problem, but not one which is insurmountable. Losing the shaman would be a grievous blow to a tribe, especially if there is no apprentice or if the apprentice also dies. Even so, it is not a situation which is unprecedented in the real world, as primitive tribes often have just the same situation, such as a griot in parts of West Africa being the sole keeper of orally-transmitted knowledge in his village. 

As for how they'd overcome the problem, the tribe would have to gather together and share their collective knowledge in a series of meetings in which questions otherwise meant for the shaman would be answered by anyone in the crowd, as well as pick a new apprentice shaman. Some answers would be contradictory, in which case the chief and the greater consensus of the crowd would decide certain controversial questions, such as what sacrifices are acceptable for the sacred flame. Much of what the shaman does would be decipherable by means of common knowledge, such as which days were holidays and what the anointing of a new chief is like.

The only serious problem is magic, which would require the lessons from another shaman to grasp. In any event, a new apprentice in a tribe without a shaman has to go to seek out the shaman of another tribe to learn magic and (if need be), reading, which the other tribe would do in exchange for a substantial payment.