Use the adverb necessarily to mean inevitably. The best man won't necessarily have to speak at the wedding reception, but he should probably have a speech ready just in case.
Necessarily also means as a logical result or consequence of some action. Your English composition grade is necessarily based on the work you turn in for the class. If you write fairly well, you won't necessarily get an A, but you just might. Necessarily is made up of the Latin roots ne, meaning not, and cedere, meaning yield. The adjective "necessary," from which necessarily comes, originally meant no backing away.