Well, he probably survives and then throws in his lot with the US along with the rest of the northern alliance. The taleban is probably rolled up a bit quicker (although I don't see how this is really possible).
This is where him surviving actually may hurt the US. Here is a guy with the romantic title of "The Lion of Panshir", his anti-Soviet record, who is hated by al-Qaeda. This is the kind of story that pulls heartstrings (and more to the point, purse strings) on capitol hill. If he becomes president of any sort of interim state, or even if he just continues as commander of a large (maybe the largest) cohesive, US-aligned force, his shadow will loom over Afghan politics. How is this bad?
He's Tajik. In OTL the percieved pro-Tajik US policies in Afghanistan (Hamid Karzai as president, etc.) is winning no favors with the Pashtuns. In fact, the "new Taliban" which is making it's presence felt in Afghanistan may be motivated as much by jilted Pashtun ethnic chauvinism as by religious fervor. If Massoud goes from being the warlord of Panshir to the warlord of Afghanistan, the Pashtuns will NOT be happy. I would say things go more or less as OTL- resentment wouldn't be that bad until it fully sunk in that the interim government was not going to remain interim for long.
The first serious departures will probably happen around Operation Anaconda. The operation could be more hard-fought than it was OTL, but more to the point, the Taleban could begin their resurgence (or rebirth, really) as an ethnic Pashtun insurgency much sooner than OTL, maybe even by up to a year. Political opposition may be a problem as well; the Afghan government may loose all support from Pashtuns if it is seen to be dominated by a Tajik strongman with the firepower and loyalty to really impose his will outside Kabul.