Van Gaal blow as last-gasp Costa denies United
Chelsea 1-1 Man United
A draw at Stamford Bridge and a vibrant attacking performance would be a huge positive for Manchester United under normal circumstances.
But these are not normal circumstances for them or for manager Louis Van Gaal, who betrayed his frustration at the end by berating fourth official Craig Pawson for the timing of the final whistle just as, six minutes into added time, added for an apparently serious leg injury suffered by Kurt Zouma, his team countered in search of a desperately-needed victory that felt so close before it was taken away.
They thought they had it. They thought they had done enough. They thought David De Gea's outstanding goalkeeping - with one emphatically world-class save to beat away a fierce Branislav Ivanovic volley - had repelled Chelsea and that they would be closing the gap on fourth-placed Manchester City to just four points.
Instead it stands at six. Not insurmountable but the margin for error is now so tight that even a result like this is damaging for Van Gaal, who knows - despite his angry rebukes - that the spectre of Jose Mourinho is looming large.
Mourinho's agent Jorge Mendes is making sure of that. Making sure that the United hierarchy know his man is ready and available to be considered should a switch be made. The pressure is on.
It would appear impossible for Van Gaal to survive into the final year of his contract as United manager if he does not secure Champions League football.
With the top four playing each other next Sunday, a day after United are away to Sunderland, and with Van Gaal still having to face that quartet, the crunch is approaching fast.
There is no margin to slip for United, just as Daley Blind slipped here allowing Costa to run on to Cesc Fabregas' clever stabbed pass and score the equalising goal.
If Blind was unfortunate there, then he was perhaps lucky not to concede a first-half penalty when John Terry's shot hit his arms.
Van Gaal admitted the "title gap is growing and growing" but this is not about winning the title. He needs that top four. That has to be the prize.
It has gone for Chelsea, the champions, with interim manager Guus Hiddink effectively admitting that afterwards.
That may also come as a relief, of sorts, as Chelsea will surely now start to turn more towards the cup competitions with the FA Cup and, yes, the Champions League still winnable for them. They are only seven points clear of relegation but will not go down.
There will be claims that Hiddink's second spell at Chelsea is, after six draws in eight league matches, lacking traction but they would have certainly lost this match not so long ago. Instead this Chelsea are now, again, fighting to the end.
And none more so than Costa. The striker carried that fight but the menace he showed was channelled properly.
There were no distracting rows, just a constant stream of aggressive intent that earned a hug and an exchange of words from De Gea once this encounter was over. And, at the end, Costa's name rang out around the stadium; his status re-confirmed.
The focus was on Terry also, of course, with this being the Chelsea captain's first home game since he announced he did not expect a new contract to be offered to him by the club and that - as things stand - he will be leaving at the end of this campaign.
Maybe Zouma's injury - if it is a bad as feared - might also impact on Chelsea's decision on Terry.
Roman Abramovich was here. He heard the songs, he saw how Terry played and he knows this could develop into awkward situation.
Almost as awkward as this fixture being relegated to 13th place hosting fifth when, for so long, it was associated with title-defining games, games that showed how the shift in power switched between these two clubs over the past decade or so before Manchester City entered that conversation.
There are a few more voices involved this season - with, gloriously, Leicester City plus Tottenham Hotspur and Arsenal hoping to last the pace - and the bottom line for both these clubs is that they need to shape up fast.
They need to work out exactly what they want to do and where they want to be next season. And that starts with a decision over who manages them.
United had dominated with Anthony Martial, again impressive, cutting inside and forcing a superb one-handed save from Thibaut Courtois with a drive destined for the top corner before De Gea then acrobatically held onto Nemanja Matic's header before Costa dragged a shot across goal and Oscar fired over.
But it was tight. It felt like one goal would be enough, and United drew two more fine saves from Courtois - who denied Rooney and Lingard before, with England manager Roy Hodgson watching, the latter scored.
Zouma's injury, hyper-extending his leg as he landed, appeared to affect Chelsea's players and when play resumed they were caught out.
United worked their way forward with the ball sent wide by Martial to Cameron Borthwick-Jackson, whose cross from the left was cushioned by Rooney to Lingard, who swivelled sharply to send an athletic right-foot shot beyond Courtois and high into the net.
It was a "beautiful" goal, Hiddink conceded.
Then came the Chelsea onslaught. De Gea denied Ivanovic, Fabregas and Costa, with a strong one-handed save before Borthwick-Jackson erred - playing Costa onside - and as Blind slipped the chance opened up.
Borthwick-Jackson slid in but it only helped Costa round De Gea and stroke the ball home.
To compound United's frustration the goal had resulted from substitute Memphis Depay losing the ball - rather than any mistake by the officials - as they attacked with four players against two defenders.
Chelsea deserved their draw, but Van Gaal had victory in his grasp.