John Fleck did his best to repair some of the damage but it was a futile exercise.
Chris Wilder's withering critique of Sheffield United's performance, coupled with Leeds' victory over Sheffield Wednesday later in the day, meant not even an impassioned call to arms from the Scottish midfielder could disguise the fact that, following a dramatic and damaging afternoon at Bramall Lane, the race for the Championship's second automatic promotion spot has swung firmly in the direction of Elland Road.
Millwall's last gasp equaliser, after substitute Gary Madine had earlier broken the deadlock, means United now trail Marcelo Bielsa's squad by three points with only four matches left to play.
"We are still in the mix and it is not a disaster," Fleck insisted as he digested the implications of this result. "We are still there, we need to try and pick up a few more wins because there's plenty to play for."
The trouble is, on the evidence of Saturday's contest, United appear inhibited rather than inspired by the prize on offer. And, although neither he nor his manager would admit it, the debris of an uncharacteristically subdued display threatens to cast a shadow over their remaining fixtures. John Egan's sending-off, for deliberate handball on the goal line before Ben Marshall's missed penalty, means he will definitely miss Friday's meeting with Nottingham Forest while Billy Sharp and Chris Basham are also likely to be absent after suffering hamstring injuries. With two of their leading names facing spells on the sidelines, United face an uphill struggle to record the quartet of wins they might now require.
"The opportunity is still there but it is hard to look at the positives because we did not win the game," Fleck said. "But I am sure, through the week, we will be ready for the next one. That's the way it's got to be."
United saved some of their most timid work of the season for one of the most important weekends on the competition's calendar. With the scheduling handing them a chance to test Leeds' mettle - Neil Harris' side slipped to 20th despite Jake Cooper's added time header - they instead produced a shift so lacking in purpose it left Wilder questioning both their ability to follow instructions and psychology.
There were smatterings of the fearless,enterprising football which has become United's hallmark under the 51-year-old, most notably when Fleck, making the most of a colleague's poor pass, punched a hole in Millwall's defence before squaring the ball for Madine to score for the third time since arriving on loan from Cardiff City. But while Wilder was furious with referee David Webb for failing to punish Ryan Leonard's foul on David McGoldrick moments before Cooper pounced, he acknowledged United's conservatism, despite being urged forward by the bench, cost them dear.
"The thing I'm struggling with is if they thought that was the best play to play," Wilder said, clearly struggling to contain his anger. "I think it was a mindset more than anything. It was a decision from players not to get forward and run forward to get up the pitch.
"We all know, if you are 30 yards from goal, it gives the opposition more chance to ping a dangerous ball in. It's a thought process more than anything else."
Although Millwall exploited United's nerves, it was impossible to escape the conclusion that Wilder's men were not the architects of their own downfall. With Jack O'Connell parachuted back into the starting eleven after recovering from a calf problem, the hosts began the afternoon brightly as Mark Duffy tested David Martin from long range and Enda Stevens and Sharp probed for openings. Sharp's departure, combined with Basham's earlier exit, inevitably affected their rhythm. The sense of occasion did too. But when Madine sliced through the tension, what should have been an opportunity to move through the gears instead saw United become increasingly constrained.
"I felt we started okay and then it turned into a scrappy game and it was always going to be tight," Fleck conceded. "We did well to get ourselves in front, but got sloppy after our goal."
Millwall, sensing United nervousness, pressed with Mahlon Romeo going close with a free-kick before Egan saw red for palming Tom Elliott's effort away from goal. United seemed to have escaped when Marshall's spot-kick rebounded back of the crossbar but when Leonard's foul on McGoldrick went unpunished, they edged back upfield and saw Cooper bundle home.
"We can't, just because Millwall deserved something, think 'They've missed a penalty and so let's not give them that free-kick.' Just give us free-kick and lets get out of here," Wilder. who confronted the officials following the final whistle, fumed.
Fleck, who together with Madine was United's most purposeful performer, accepted his manager's complaints.
"We did not compose ourselves enough on the ball, going forward," he said. "That is probably why we ended up getting penned back as we were giving it away too often. They were just banging balls into the box and that is what they are good at.
"We are used to playing a certain way. But probably in the last two or three games, we have not played as well, but still got results. There is still a long way to go and we just need to look forward to the next game."
Sheffield United: Henderson, O'Connell, Egan, Basham (Madine 21), Stevens, Freeman, Norwood, Fleck, Duffy (Coutts 72), Sharp (Cranie 65), McGoldrick. Not used: Moore, Dowell, Hogan, Stearman.
Millwall: Martin, McLaughlin (Marshall 58), Cooper, Wallace, Thompson, Ferguson, Romeo (O'Brien 81), Pearce, Tunnicliffe, Leonard, Gregory. Not used: Amos, Meredith, Williams, Elliott, Morison.
Referee: David Webb (Lancashire).
Attendance: 26, 703.