As Sunderland walked off the pitch at Bramall Lane, beaten but unbowed, and applauded by the travelling fans, the step up to the Championship was thrown into stark relief. The margin for error in the second tier is vanishingly small.
A red card, a defender switching off at a set-piece, a bad refereeing decision, an unlucky deflection, a moment of inspiration from an opponent, a goalkeeper lumbering forward to score a one-in-a-thousand header, these are the type of key moments that can tip the balance at any level. But whereas in League One, where Sunderland have spent the last four seasons, you might get away with it, in the Championship better teams and better players will take full advantage.
That is not to say Sunderland will always be on the receiving end - far from it - but they were on this occasion. Alex Neil said before the game that Sheffield United, a side that spent two of the last three seasons in the Premier League, that finished in the play-offs last term, and is again one of the favourites for promotion, would be Sunderland's most difficult test to date.
READ MORE: Sunderland players will rally around Dan Neil after his red card, says Lynden Gooch
It was going to be tough enough, without doing the Blades any favours. So it was no surprise that Dan Neil's red card for a professional foul after half-an-hour proved to be the gamechanging moment.
Up to that point, Sunderland had been more than a match for their hosts and had looked a threat. But when Neil's bad touch let in James McAtee, and the 20-year-old responded by dragging down the on-loan Manchester City man and was shown a straight red card, the writing was on the wall.
Within two minutes, the Blades went in front through Anel Ahmedhodzic from a set-piece, and it became - for a time, anyway - virtually one-way traffic. Max Lowe's goal just after half-time gave Sheffield a two-goal cushion and after that Alex Neil had to try something different, switching to a back four and taking off Ellis Simms as part of a triple substitution.
It immediately paid off, with a mistake from former Sunderland academy man John Egan punished as Jay Matete and Ross Stewart combined to send Lynden Gooch through on goal and he halved the deficit with a delicate chip. But Sheffield soon wrested back control, with only the width of a post denying Sander Berge a goal, while Anthony Patterson made a string of fine saves late in the game to keep the score down.
This was Sunderland's first defeat since returning to the Championship, and it ended their 19-game unbeaten run in the league and play-offs stretching back to mid-February. Neil said afterwards that he was proud of his players, and the Sunderland fans who had made the trip to South Yorkshire clearly agreed.
And there was much to be proud of - the way Sunderland had played in that first 30 minutes, the fact that they did not collapse despite playing for an hour while a man down, the character they showed to come back from 2-0 down to get back in the game and at least test Sheffield United's nerve. All of this augurs well for the remainder of the season.
But Neil also left Bramall Lane with some new worries. He went into the season confident that he had assembled a competitive first-team but concerned about the lack of strength in depth in his squad, and in certain positions in particular.
Since then, summer signing Daniel Ballard has been ruled out for the forseeable future with a fractured foot, Carl Winchester has suffered a back issue, and last night influential midfielder Corry Evans missed the game through injury while centre-back Danny Batth came off as a precaution with a slight groin problem. Dan Neil, of course, will miss this weekend's trip to Stoke through suspension, although at least he will be absent for just one game.
The transfer window closes a fortnight today, and the trajectory of Sunderland's season could hinge on the signings that arrive before the deadline. As I say, the margin for error in the Championship is vanishingly small - off the field, as well as on it.
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