There is No Explicit Proof that Mahdi Will Lead Prophet Isa (as) in Prayer

[Rationalist]

Firstly there are no mutawatir hadith in regards to the Mahdi. Secondly, with the hadith we have, there is no proof that Imam Mahdi will even survive till the return of Prophet Isa (as). The Mahdi will rule for 7-9 years max, which means its doubtful he will survive till the coming of Prophet Isa. Thirdly, Mahdi is not the only Imam who will come to guide the Ummah toward the Sunnah. After Imam Mahdi another Imam by the title of Qahtani who will appear and become the Calipah and Imam of the Muslims.

Lastly, the hadith presented in Sahih Muslim says the Imam among you will lead the prayer. It doesn't specify the Mahdi. Instead some of the narrators inserted the name Mahdi  in the hadith when the original text did not have it.

Here is the original hadith.

It is narrated on the authority of Abu Huraira that the Messenger of Allah (ﷺ) observed:
What will be your state when the son of Mary descends amongst you and there will be an Imam amongst you? (Sahih Muslim)

[Hani]

Why does it have to be Mutawatir? : )

[Rationalist]

It doesn't. However, for the 12ers you must believe in the Mahdi otherwise you are non-Muslim.