Ander Herrera scored one goal, set up another and executed the perfect man-marking job on Eden Hazard to set up Manchester United's 2-0 win over Chelsea that opened up the Premier League title race on Sunday.
United didn't even need rested top scorer Zlatan Ibrahimovic to consign the leaders to their fifth league loss of the season, which left them only four points ahead of Tottenham with six games left.
Marcus Rashford — Ibrahimovic's replacement up front — ran onto Herrera's through-ball to give United a seventh-minute lead. A constant menace for Chelsea's defense all game, Rashford ensured Ibrahimovic wasn't missed at Old Trafford.
Herrera shadowed Hazard in a tactical plan by United manager Jose Mourinho that limited the effectiveness of Chelsea's main attacking threat. On a rare occasion he left Hazard's side, Herrera popped up in Chelsea's area in the 49th minute to smash a deflected shot high into the net as United saw out the win easily.
Simon Johnson assesses the key talking points at Old Trafford...
Rashford proves what an exciting talent he is
Chelsea may have been relieved to see Zlatan Ibrahimovic named on the bench, but perhaps they would have preferred facing the ageing striker.
For in his stead was the more agile and speedy Marcus Rashford and they just couldn't cope.
Naturally his goal in the seventh minute, his first in the League at Old Trafford since September, grabbed the headlines.
But the 19-year-old gave Manchester United's attack the cutting edge it has lacked so often this season.
Ibrahimovic has been superb in front of goal and is rightfully one of the Player of the Year candidates this term.
But at 35, he can't run in behind defenders like Rashford can and Chelsea duo David Luiz and Gary Cahill were caught out time and time again.
England manager Gareth Southgate wants the Manchester United trainee to be part of his plans - on this evidence you can see why.
The Manchester United fans made their appreciation known loud and clear when he was subbed late on for Ibrahimovic.
Costa has become a liability for Chelsea
Once a candidate for Chelsea's player of the season, Costa is now fast becoming their weakest link.
It is bad enough he has scored just three League goals since the turn of the year, but he is also failing to lead the line with any conviction.
On so many occasions, a promising move by Chelsea broke down because he was either knocked off the ball easily or a pass went wayward.
In the first half of the campaign, Costa used his aggression for positive reasons, to bully opposition defenders and make life difficult.
But the Spain international has resorted back to the player that gets involved in unnecessary skirmishes and looking like a red card waiting to happen.
Referee Robert Madley's patience was wearing thin from a very early stage and it was no shock to see him booked in the first half.
Hazard can't escape Herrera's shadow
The last time these two players faced each other on the pitch, Herrera was given his marching orders for two cynical fouls on the Belgian.
Chelsea took advantage to knock Manchester United out of the FA Cup, yet Herrera went about his job far more effectively this time.
Granted there were still a few fouls in there, but not quite ugly enough to get in Madley's bad books until 18 minutes from the end, when he received a yellow card. But the damage to Chelsea had been done by then.
If anything, Herrera shackled Hazard even closer than at Stamford Bridge and Chelsea's best attacking outlet was completely nullified.
On one occasion that Herrera forgot plan A and left his man in space, manager Jose Mourinho shouted loudly to remind him.
The Spain international then capped a fine defensive performance by scoring the crucial second goal just after half-time too.
Mourinho has the last laugh on his former club in 2016-17
Chelsea fans took great delight in taunting Mourinho this season when Manchester United lost in the League and Cup at Stamford Bridge.
'You're not special anymore' they cried, but Mourinho made a point of saying that until someone beats his record of three League titles as Chelsea manager, he will remain No.1.
Those in the away end were given another reminder of Mourinho's abilities this afternoon as he inspired his players to arguably their best performance of the campaign.
In the build-up to the match, the Portuguese was at his most sombre, effectively saying his team had little chance due to injury problems and the fact Manchester United had played just three days earlier.
Anyone connected to the west London club should have recognised it for the mind games it was - after all, he used it to good effect on so many occasions when with the Blues.
In many ways, this brought back memories of when Liverpool's party was ruined by Mourinho's Chelsea in 2014. This time his old club were on the receiving end.
Conte faces his biggest week as Chelsea manager
The Italian has proved to be an outstanding coach since joining last summer, yet now he comes his biggest test.
This was as bad as Chelsea have played since he took over. It brought back memories of the terrible showing against Arsenal in September.
Back then the defeat wasn't disastrous because there was still plenty of games left to put things right - and he did just that with a remarkable 13-game winning run.
However, this reverse has come at a terrible time - when the finishing line in the title race is in sight.
Their second setback in four matches means Tottenham are just four points behind with six games remaining. To put it in context, their lead was 13 points after winning at Stoke last month.
What will be more concerning though was the performance. Chelsea players looked like they were defeated before kick-off.
Admittedly the loss of Thibaut Courtois to an ankle injury in training and Marcus Alonso to a knock in the warm-up was unfortunate.
They can also rue Madley not spotting a blatant handball by Herrera before Manchester United broke for the opening goal.
Still, the lack of cohesion, organisation and will to fight for the remaining 83 minutes was alarming.
The FA Cup semi-final against Spurs on Saturday was already a huge game, but now it has even more significance.
Such is the psychology of the sport, that the winner of that game will see themselves as favourites to win the Double.
Mauricio Pochettino has all the momentum on his side right now. Conte must come up with a plan to reverse it.