Because the answers we've been seeking all season were there all along. Because the players have it in themselves to do more than show up dumb squibs week in week out. Because this team is not as far off the pace as we've been led to believe.
In theory though, it comes down to one quiet statement Louis van Gaal made in his post-match interview.
''Tottenham are are team that like to play and so we had a lot of space..''
It perhaps affords an explanation as to why United, despite their faults, have not really been toasted by any of the bigger sides in the Premier League. It equally baffles that we've been toasted by Leicester City (of all teams) and similar teams of that kind have caused us all sorts of problems. It is therefore not illogical to conclude that United are still short on how to unlock teams that sit back in banks of 4 and hit on the counter.
In what turned out to be our best performance of the season, Louis van Gaal lined up United in perhaps the only shape that suits the talents of most of the players we have at our disposal. The suspension of Di Maria allowed him to name Juan Mata in the same team with Ander Herrera, effectively catering for the creative department whilst Fellaini and Rooney up top mixed Agriculture and finesse to deadly effect. With Carrick dictating play from deep, United were able to play with most players in their best positions hence ridding the side of the functionality and rigidity that has hand-cuffed us for swathes of the season. Mata played inside right in a way that is not akin to the traditional wing play of looking for the touchline and making a cross but in a technically astute way that provided an outlet for the midfield when it was crowded and then driving back in field diagonally to find spaces up top. It allowed for a variation of play between our wings and the midfield and as a result Spurs quite simply couldn't cope.
Questions will now be asked as to whether United are better off without another big name, Di Maria in the side. The starting XI vs Liverpool on Sunday cries out for the similar 4-1-4-1 formation we saw last night. Juan Mata certainly merits a place in the side again but at whose expense? LVG will certainly not want to chance the winning formula that has brought him his first excellent victory and performance rolled in one. Spurs were awful, but making them as such still takes some doing.
Wayne Rooney is an instinctive striker and he's good at it. His understanding of most roles on a pitch doesn't make him good at every role so you really wonder if the manager will make the mistake of entrusting him with midfield duties that are proper suited to better players in those positions. His midfield appearances were in a period when the 'twitterratti' bayed for a run of games for Radamel Falcao. Now that that thirst has been quenched, it's a no brainer that we should make use of the best goal-scoring talent the club has at it's disposal to maximum effect in this of all periods.
The only downside on the night was the accident waiting to happen that is our defence. Phil Jones has somehow fallen down the pecking order to below Chris Smalling. He's only second to Jonny Evans when it comes to how many nerves they can bag. The game was hardly a minute old when Jones gave a nervy, ill-timed, ill-directed back pass to David De Gea. It is only last week on Monday that we had the back pass debate on this blog and yet Jones quickly set United into the back pass mindset at the very first opportunity and with limited accuracy at that. He improved as the game wore on, but boy oh boy do we need some certainty in that department.
Chris Smalling was asked to keep tight on Harry Kane and boy did he do just that, and then some. Not many centre halves in the league have managed to keep the budding talent at bay but it seems Chris shall be handing out memos to defenders around the country on how to deal with England's next big thing.
Daley Blind was surprisingly untroubled by Andros Townsend for the opening half hour on that left hand side but wit appeared to be largely because Fellaini and Young tag teamed effectively to cause Spurs all sorts of problems on that wing.
Make no mistake about it though, this was a much needed win. It's hard to imagine how United would take in the rest of the season if we had lost our second straight game against top opposition with even bigger tests ahead. But we'll worry about that on the weekend. For now, we can only imagine ourselves laying claim to that second spot that has suddenly come into view. With all sides above and directly below United providing the opposition between now and May, you can see how United can easily dictate the shape of the final league standings, both to their detriment and advantage. It's at least reassuring that we shall kick-off at Anfield still in 4th place, and still in charge of our destiny.