George Q. Cannon discusses how Sidney Rigdon was the prophesied "spokesman" for Joseph in the Book of Mormon.
George Q. Cannon, "Predictions in the Book of Mormon—Evidence of Its Divinity—Proof that Joseph Smith Was Inspired—Predictions Concerning the Indians Fulfilled—Coming Forth of the Book of Mormon Foretold—Plainness of Its Teachings—Prediction Relating to Sidney Rigdon—Only Two Churches—Other Prophecies Being Fulfilled," April 6, 1884, Journal of Discourses, 26 vols. (Liverpool: John Henry Smith, 1884), 25:127
Those who knew Sidney Rigdon, know how wonderfully God inspired him, and with what wonderful eloquence he declared the word of God to the people. He was a mighty man in the hands of God, as a spokesman, as long the prophet lived: or up to a short time before his death. Thus you see that even this which many might look upon as a small matter, was predicted about 1,700 years before the birth of the Savior, and was quoted by Lehi 600 years before the same event, and about 2,400 years before its fulfillment, and was translated by the power of God, through his servant Joseph, as was predicted should be the case, and at a time, as I have said, when there was not a man upon the earth who was a member of the church of Jesus Christ of Latter-day Saints. The church had not yet been organized, and Joseph did not know, unless he knew by the spirit of revelation, whether any man would receive the Gospel. I doubt whether he knew as to how the church would be organized. He had some idea, doubtless; but there were many things which he himself did not know, till he wrote this translation.