This hadith is weak. Narrated by Ibn Majah and Al-Hakim, and weakened by Al-Bayhaqi and Al-Hakim. It states: "Abu bin Salih, and he is a fabricator of hadith." (Tuhfat al-Ahwadhi). The hadith revolves around Muhammad bin Khalid Al-Jundi. Al-Dhahabi said: Al-Azdi said: "He fabricates hadith." Al-Hakim said: "He is unreliable." Al-Hafiz said in Al-Taqrib: "He is unreliable." The hadith was weakened by Ibn Taymiyyah in Minhaj al-Sunnah and Al-Albani in Al-Da'ifah.
If we want to reconcile, it can be said, as the scholars have said, that the hadith: "There is no Mahdi except Jesus" is weak, as previously stated. And assuming its authenticity, the meaning of the hadith would be: "There is no perfect Mahdi except Jesus." And there is no doubt that Jesus - peace be upon him - is more perfect in guidance than the Mahdi. Therefore, the negation intended in the hadith is a negation of perfection, not a negation of existence. This is assuming the hadith is sound; rather, it is weak, as you have seen. Praise be to Allah, by whose grace good deeds are completed.
This does not mean that it is a text that negates the Mahdi and denies his emergence.
There is no single hadith that denies the Mahdi. Rather, it is the complete opposite. The number of hadiths that affirm the matter of the Mahdi has reached the level of mutawatir (mass transmission), which is only rejected by one of three people: the first is the ignorant, the second is the stubborn, and the third is the envious. So, be a knowledgeable Muslim brother. Ignorance is a flaw, but with knowledge, it is removed. Do not be stubborn to the command of Allah, and do not be envious of the Mahdi, lest you deny him and perish. Likewise, our religion is taken from the Book and the Sunnah, so nothing will lead you astray except through them.
As for the hadith: "There is no Mahdi except Jesus." It is not authentic, and it is not permissible to use it as evidence to deny the Mahdi.
Most of those who narrated the hadiths of the Mahdi are from the Shiites. This is one of the doubts that have been spread, and it has no truth to it at all. Only a fabricator or an ignorant person would say this.
Therefore, most of the later researchers into the hadiths of the Mahdi did not have a single source from the Shiites. As for the later ones, if they took from the Shiites, they either took what had an indication from the Sunnis, or it agreed with the truth and did not contradict the narrations of the Sunnis. Most of this falls under the category of the Mahdi's attributes or the signs of his emergence.
It is permissible according to scholars to act upon a weak hadith regarding the matter of the Mahdi, and its mutawatir status is not a condition.
This is something that the scholars of Islam have agreed upon. Those who said this include: Ahmad bin Hanbal, Zayn al-Din al-Iraqi, Jalal al-Din al-Suyuti, Imam al-Shawkani, Ibn Hajar al-Haythami, and many others.
Finally: Allah the Almighty has argued with the Jews and Christians concerning the texts they have from the Torah and the Gospel, which they have altered, and has declared them infidels when they denied the truth of the Messenger of Allah Muhammad bin Abdullah. So what can we say when we have some hadiths and narrations with weak chains of transmission only?!
The hadiths of the Mahdi (peace be upon him) have reached more than mutawatir. It is not hidden that all the hadiths of the Mahdi, peace be upon him, are from the news of the unseen. The Companions were not inspired. This means that the hadiths that are attributed to some of the Companions were actually from the Messenger of Allah, peace and blessings be upon him and his family. It is logically impossible for a number of more than fifty Companions to agree on a lie! Especially since they had no interest in lying, and no one forced them to do so. Furthermore, they did not know the identity of the awaited Mahdi, peace be upon him, with certainty, so that they would lie for his sake. All these Companions testified and heard the Messenger of Allah, peace and blessings be upon him and his family, narrating the hadiths of the Mahdi, peace be upon him, and among them, by way of example:
The People of the House of Prophethood and the Family of Muhammad, peace be upon them: If there were no narrators for the hadiths related to the Mahdi other than these, they would have been sufficient and certainty would have been achieved through them.
From the wives of the Prophet, peace and blessings be upon him and his family: The Mothers of the Believers, such as Umm Salamah, Aisha, and Umm Habibah.
From the Banu Hashim: Such as Al-Abbas bin Abdul Muttalib, the uncle of the Prophet, Abdullah bin Al-Abbas, and Abdullah bin Ja'far Al-Tayyar.
Caliphs and candidates for the caliphate: Such as Umar bin Al-Khattab, Uthman bin Affan, Abd al-Rahman bin Awf, Talhah bin Ubayd Allah, Abdullah bin Umar bin Al-Khattab, and Abdullah bin Amr bin Al-Aas.
The righteous Companions: Such as Salman Al-Farisi, Abu Dharr Al-Ghafari, Ammar bin Yasir, Jabir Al-Ansari, and Hudhayfah bin Al-Yaman.
A group of Companions: Such as Abu Ayyub Al-Ansari, Zayd bin Arqam, Abu Sa'id Al-Khudri, Sahl bin Sa'd Al-Sa'idi, Hudhayfah bin Asid, Mu'adh bin Jabal, and many others.