I think that this discussion is making the key point. My first reaction was Holiday, but it really could be any of White, Horford, or Porzingis also. Any one of these guys could easily be expected to be "that guy" in a specific game or for the series. BOS has multiple guys who have been good / are good enough to be that guy, just by playing as good as they have in the past. If any one of them is that guy, it would be no surprise or statistical abnormality.
But do you really expect any of the DAL second tier to do that? Washington? Jones? Gafford? (the other 3 starters). It is possible that Jones or Washington could get hot from 3 for a stretch and cause some problems. Role players seem to have a history of doing that vs. BOS in the playoffs recently. But if it happens, it will be an outlier for that player, not something that is reasonably expected.
And keep in mind that it will probably take more than 1 of these guys to have that kind of series for either team to win. So if it is unlikely or improbably that 1 of the DAL second tier players have a "Cinderella" series, how likely is that more than 1 will? As compared to BOS, how surprised would everyone be if say both White and Porzingis have a big series? If all our second tier guys are even solid, I think we take the series, DAL would seem to need some or all of their second tier guys to be better than their norms, better than solid. But that is exactly how MIA beat us last year.