The term “messenger” used while referring to the coming of the blessed Mahdi is not meant in the sense of “prophet.” As revealed in the verse “… [Muhammad is] the Messenger of Allah and the Final Seal of the Prophets” (Surat al-Ahzab, 40) our Prophet Muhammad (saas) is the final prophet sent by Allah to mankind.
The term “messenger” used in reference to the blessed Mahdi, who we are told will appear in the End Times, is employed in the sense of “communicator of the word,” as revealed in the Qur’an. In the Qur’an, each and every Muslim is held responsible for performing this duty of communicating Allah’s word. In this sense, therefore, every Muslim is a messenger of Allah. Like all Muslims, the blessed Mahdi is in the position of a communicator carrying out that duty. He calls on people to display excellent moral virtues, admonishes with the Qur’an, and tells them things about the Qur’an of which they are unaware. He is a means by which Islamic moral values will come to prevail on Earth and will assume the spiritual leadership of all Muslims.
However, the role of messenger to be assumed by the blessed Mahdi is not one bestowed by way of revelation. His is not the role of messenger revealed by Divine revelation, but of communication of the word of Allah. It is in any case impossible for anyone to claim that he is a revelation-based messenger in the Qur’anic sense unless it is revealed by Allah. Since it will not be so revealed, and since no other Divine Book will be sent down after the Qur’an, such a thing is completely out of the question.
But every Muslim is a messenger in the sense of being a communicator of the word. The word “messenger,” meaning “an envoy who preaches the faith of Allah,” can also be used to refer to the blessed Mahdi in that sense. Like all Muslims, the blessed Mahdi is a messenger of Allah charged with preaching the message of the Qur’an. He is one who tells people of Allah and the Qur’an. In that sense, the blessed Mahdi may be referred to as a messenger, in the same way as all other Muslims.