Match ends, Manchester United 2, Chelsea 1.
Manchester United moved back into second place in the Premier League as substitute Jesse Lingard gave them victory over top-four rivals Chelsea at Old Trafford.
The Red Devils had to come from behind after Willian started and finished a flowing move to give the visitors the lead in the first half.
United pulled level before the break, with striker Romelu Lukaku getting in front of Marcos Alonso to score against his former club for the first time.
Lukaku then set up Lingard to head home with 15 minutes left, after which Chelsea's Alvaro Morata had a goal incorrectly disallowed for offside.
Defeat leaves the Blues in fifth place after Tottenham beat Crystal Palace 1-0 earlier on Sunday.
A run of just one win in their past four league matches, coupled with Spurs' 10-game unbeaten streak, means they are out of the Champions League places with 10 games remaining.
Chelsea went close to taking the lead at Old Trafford after just four minutes, when Morata ran on to Alonso's brilliant volleyed cross, but his shot hit the bar.
Lukaku, facing the club he played for between 2011 and 2014, also had a flying volley superbly saved by Thibaut Courtois.
Lukaku rises to the occasion
Lukaku came into the game having scored 12 goals in the Premier League so far this season, but with none - and only four shots on target - against the other teams in the top six.
The Belgian, signed for £75m from Everton last July, had never scored in either seven previous league appearances against Chelsea, or 10 for the Blues during a spell in which he enjoyed more success while on loan at West Brom and Everton.
But the 24-year-old edged in front of Alonso to finish off a neat move involving Alexis Sanchez and Anthony Martial and end his dry spell against the top flight's elite.
Lukaku came close to a second when he met Sanchez's superb ball with a flying left-footed volley, but Courtois denied his international team-mate by tipping the ball over.
He swung the match decisively in United's favour, though, when he slipped free on the right of the box and chipped in a cross that Lingard nodded home.
Missed opportunity for Chelsea
When Willian rounded off a move he started to score his fourth goal in his past three outings, everything looked to be falling into place for Antonio Conte's side. The Blues had not lost after scoring first in the league since a 2-1 defeat by Crystal Palace on 1 April in 2017 - 22 games ago.
But things unravelled - and while Conte substituted the influential Eden Hazard, Mourinho's decision to bring on Lingard was rewarded.
Conte will be aggrieved by the decision to disallow Morata's late goal, and the Italian will look back at the visit to Old Trafford as an opportunity missed.
Peace breaks out between bosses
Much of the build-up to the match had centred on a verbal spat between the two managers over the preceding weeks.
Mourinho had referenced Conte's four-month ban for failing to report match-fixing, for which the Italian was later cleared of any wrongdoing.
The Chelsea boss called his United counterpart "a little man" - though both played down the issue in their final news conferences before the match.
Conte said the dispute had "stopped", while Mourinho said he would "not speak about it - not at all".
Despite that, there was a frisson of interest about how - or perhaps if - they would greet each other before kick-off.
Mourinho was first out of the tunnel while Conte delayed his entrance slightly, but the pair shook hands before referee Martin Atkinson got the match under way.
There was another handshake at full-time, coupled with a conciliatory pat on the back of the head from Mourinho.
Man of the match - Romelu Lukaku (Manchester United)
What the managers said
Manchester United manager Jose Mourinho: "It is not special because it is Chelsea, it is very special as we beat the champions, who are a fantastic team, who are very difficult to beat and because this is the three points that keep us in second place, which is the place we are fighting for.
"It opens a little gap, not much to Chelsea, and it is a very important result for us. It feels special as the players gave everything. It is not possible to win against a team with the quality of Chelsea without the quality we gave.
"We did not start well, Chelsea started much better, but we found our position on the pitch and found our concentration and we grew up throughout the game."
Chelsea manager Antonio Conte: "We had a great chance to get a good result against Manchester United, but instead we talk about a defeat.
"You must be the best team at the end of the game, not just in the first half. We had a great chance to take three points here. Now we have to start to work this season as the league is very difficult and we need to fight for a place in the Champions League. I hope the players realise this soon.
"Luck is important for every team, but this season we have not been so lucky. It is disappointing after this performance to finish with a defeat."
Lukaku and Lingard lead way - the stats you may have missed
- Lingard has scored 13 goals in all competitions for United this season - only Lukaku (22) has scored more.
- United have won two of their past three matches against Chelsea - one more than they managed in their previous 14 combined against the Blues.
- Chelsea have lost back-to-back away Premier League matches for the first time since November 2015, when they were managed by Mourinho.
- The Red Devils have lost just one of their past 35 home league matches against the defending top-flight champions (W20 D14).
- Chelsea lost a Premier League match in which they scored first for the first time since last April against Crystal Palace. They had won 22 games consecutively when scoring first before today.
- Willian has scored 12 goals in all competitions this season, equalling his best tally in a season for Chelsea (also 12 in 2016-17).
- Eden Hazard has been involved in nine goals in his past eight appearances for Chelsea in all competitions (six goals, three assists).
- Chelsea have only lost four of their past 55 Premier League games played on Sunday - all four defeats have been in Manchester (two against United, two against City).
Chelsea return to the north west when they take on Manchester City at Etihad Stadium on Sunday, 4 March at 16:00 GMT. United are in action the day after, when they face Crystal Palace at Selhurst Park at 20:00.
- 1de Gea
- 25A ValenciaBooked at 68mins
- 31MaticBooked at 85mins
- 7SánchezSubstituted forBaillyat 81'minutes
- 11MartialSubstituted forLingardat 64'minutes
- 40Castro Pereira
- 15MosesSubstituted forGiroudat 78'minutes
- 7KantéBooked at 30mins
- 6DrinkwaterSubstituted forFàbregasat 81'minutes
- 10E HazardSubstituted forPedroat 73'minutes
- 9MorataBooked at 88mins
- Martin Atkinson
- Shots on Target
Second Half ends, Manchester United 2, Chelsea 1.
Corner, Chelsea. Conceded by Antonio Valencia.
Foul by César Azpilicueta (Chelsea).
Paul Pogba (Manchester United) wins a free kick on the left wing.
Willian (Chelsea) wins a free kick on the right wing.
Foul by Paul Pogba (Manchester United).
Attempt blocked. Romelu Lukaku (Manchester United) left footed shot from the left side of the box is blocked. Assisted by Jesse Lingard.
Attempt blocked. Olivier Giroud (Chelsea) left footed shot from outside the box is blocked. Assisted by Álvaro Morata.
Delay over. They are ready to continue.
Delay in match Nemanja Matic (Manchester United) because of an injury.
Willian (Chelsea) wins a free kick in the defensive half.
Foul by Ashley Young (Manchester United).
Álvaro Morata (Chelsea) is shown the yellow card for a bad foul.
Foul by Álvaro Morata (Chelsea).
Chris Smalling (Manchester United) wins a free kick in the defensive half.
Offside, Chelsea. Cesc Fàbregas tries a through ball, but Álvaro Morata is caught offside.
Nemanja Matic (Manchester United) is shown the yellow card for a bad foul.
Willian (Chelsea) wins a free kick in the attacking half.
Foul by Nemanja Matic (Manchester United).