Amir, you can ask the Gemara the same question. Why Daniel and not M”R or D”HM. Note that Mashiach is from shaivet Yehuda and M”R Rabbainu is from shaivet Laivi.
The answer might be that if one accepts what Rambam says Mashiach will do, along with him also coming min hamasim, he will lead a Teshuva initiative at the end of galus for example. And based on the language of Rambam it is in a natural manner, as he says “he will ‘compel’ all of Binai Yisrael to go [in the ways of Torah]”. So one may purport that it is not happening at the time of his Techiya. But even the Teshuva thing doesn’t confirm him definitely the Mashiach, it is simply a condition to assume that someone is.
So if one is presuming a Mashiach min hamasim..
Short of completing the tasks that constitute the actual culmination of his coming (building the B”HM and bringing all Yiddin to E”Y) all the preliminary identifiers (learns Torah and performs Mitzvos like D”HM, promotes Teshuva..) one would argue are his original initiative in life, and as implied happen at the end of galus. Yet being these tasks do not confirm him 100% yet, he would still be the essence of “min hamasim” if he does the two last tasks after his Techiya.
So saying the Rebbe will be the one min hamasim follows the spirit of the Gemara pointing to Daniel as a min hamasim example… by identifying relevant characteristics in his life related specifically to Mashiach (in his case Rambam’s characterization), and not simply applying one’s self as the measurer of the piety of the various righteous, which seems preposterous. But rather again, based on qualifiable factors of the person’s life directly related to the initiative of Mashiach.
Footnote: based on the teaching that “even the smallest among you can revive the dead”, it may be that identifying the actual Mashiach based on the fulfilment of prerequisites (during his life), as Rambam brings, empowers his soul, and actually contributes to his Techiya, bringing about the building of the B”HM etc. Another level of “bringing Mashiach”.