José Mourinho’s Manchester United are not the first side to defeat Antonio Conte’s Chelsea or even the first side to defeat Conte’s Chelsea since their season-changing switch to 3-4-3 back in October but no other side has nullified Chelsea’s attacking threats so effectively. This was the first time for a decade that Chelsea did not have a shot on target in a Premier League match. Suddenly, with Tottenham only four points behind, the title race is back on and Chelsea’s remaining opponents have a tactical template to follow.

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Mourinho’s primary strategy was simple: man-marking Eden Hazard, Chelsea’s dangerman and the player he fell out with so spectacularly last season. This should not have come as a surprise, considering Mourinho used this approach in the recent 1-0 FA Cup quarter-final defeat at Stamford Bridge, when Phil Jones was deployed in that role but Ander Herrera was dismissed for fouling Hazard when the Belgian drifted into his zone. Herrera was given that responsibility on Sunday and played the role to perfection. Only one player in the Premier League, Middlesbrough’s speedy winger Adama Traoré, has dribbled past opponents more frequently than Hazard this season, yet at Old Trafford he did not attempt a single dribble. He was starved of possession, starved of space and Herrera deserves great credit for his diligence.

It was peculiar, however, that Hazard and Chelsea did not have an obvious plan for combating this man-marking approach, considering Mourinho had showcased these tactics in that FA Cup meeting. Indeed, while Mourinho complained that Herrera’s dismissal was the turning point in that match, with some justification, Chelsea had grown into the contest before his red card, precisely because Hazard and Willian drifted around, swapped positions, and confused United’s man-marking.

At Old Trafford Hazard did not show enough tactical intelligence to drag Herrera into uncomfortable positions and Chelsea’s attacking play suffered badly. Conte’s decision to switch wing-backs César Azpilicueta and Victor Moses after half an hour, putting the latter to the left, was seemingly to provide more attacking intent down that flank and to let Hazard drift around.

Herrera was detailed to shut down Hazard, but Matteo Darmian, Ashley Young and Antonio Valenica all had a part to play in denying Chelsea space to create. Photograph: Guardian

Herrera’s man-marking role meant he played little part in Manchester United’s attacking play, so it was unfitting that he played the crucial pass for their opener. He intercepted a pass intended for Hazard – with an outstretched arm – before curling a wonderful ball in behind the Chelsea defence for Marcus Rashford to sprint on to and beat the advancing Asmir Begovic.

This justified Mourinho’s decision to start Rashford ahead of Zlatan Ibrahimovic – in the FA Cup game, with the Swede suspended, Rashford worked the channels effectively and outwitted David Luiz but missed his one key chance. This time he played similarly, had more support with Jesse Lingard pushed forward into a second striker role and underlined his potential to become an exceptional all-round striker. At the start of the second half Herrera’s deflected shot extended United’s lead.

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After the second goal Conte changed things, introducing Cesc Fàbregas for Moses and asking the Spaniard to play at the top of midfield. This briefly caused United confusion, as Fàbregas was playing in Herrera’s natural zone, and he suddenly looked worried about the threat of stopping both Hazard and Fàbregas. Five minutes later, however, Mourinho introduced Michael Carrick for Lingard, moving from 4-4-1-1 to 4-1-4-1, and Carrick focused on nullifying Fàbregas. At times this was a total man-marking exhibition, with Mourinho seen frantically shouting at the left-back, Matteo Darmian, to stick tightly to Willian, another substitute, in the closing stages. Such strict man-marking is relatively rare in the modern game but Sir Alex Ferguson used Jones in that role against key dangermen – Gareth Bale, Marouane Fellaini, Cristiano Ronaldo – in his final season and Louis van Gaal’s midfield pressing strategy often involved nothing more than three man-marking jobs. United, more than most sides, are accustomed to that approach, although it is doubtful whether it is a viable long-term strategy.

The more pressing question is how Chelsea respond to the defeat. The game underlined their reliance on Hazard for attacking inspiration, with Diego Costa seemingly focusing on his squabble with Marcos Rojo, Pedro also subject to tight marking and little creativity from midfield before the introduction of Fàbregas. Conte can point to the absence of Thibaut Courtois in goal and the left wing-back Marcos Alonso to explain his side’s sluggishness but title winners should be capable of coping without two first-choice players. Their run-in looks relatively simple but Hazard can expect more problems with man-marking after the success of Mourinho’s approach here. Conte, so revered for his tactical intelligence this season, must find a solution.