Look - despite what Rangers fans that still claim Mark Messier as their own despite him being wildly more successful in Edmonton might lead you to believe - sports guarantees are stupid. They are stupid because they are usually don't even contain the word "guarantee" and are just a show of confidence in a team that is behind the fighting an uphill battle at the moment, and they are stupid because sports are a beast so fickle that it makes hormonal women seem predictable by comparison.
Regardless of him declaratively (and desperately) trying to speak a win into existence, P.K. Subban doesn't have a damn clue what's going to happen on Saturday night. A very resilient, very experienced team is going to take the other end of the ice in Nashville, and believing that they have no chance at winning would be just as silly as believing that Pekke Rinne could maintain his .976 save percentage from the first round of the playoffs. There is simply too many external factors - none the least of which is the NHL's proclivity to get in it's own way worse than a visually-impaired person trying to navigate a 'House Of Mirrors' - to conclude that undying desire will decide who triumphs in a competition between dozens of proud athletes.
That said, if the whole point of a seven game series is to balance out the bounces in determining who the better team is then the Predators are due to have the odds fall in their favor. This harmless promise is still dumb, but it's the least of the dumb considering the way the ice has been tilted. Pittsburgh has won two 60 minute games in about 6 collective minutes, so if I were betting man I would probably pick Nashville simply because of whatever it is that I actually learned in the 'Statistics' class that I barely passed. Assuming their goal doesn't maintain it's negligent tending, proactively proclaiming upcoming victory against a team that must be internally laughing at the fact they are up 2-0 isn't the most egregious act of presumptuous assurance. Even if it is baseless at best, and dependent upon the blind whistleblowing of the NHL's officiating at worst. As long as the Penguins aren't ironically the ones 'catfishing' the Predators with their lengthy dormant stretches, then P.K. Subban's "guarantee" is safer than most made by a member of a team in an 0-2 hole against the defending champions.