Match ends, Manchester United 3, Newcastle United 2.
Manchester United fought back from two down to beat Newcastle and avert a defeat that would have cranked up the pressure on manager Jose Mourinho.
The hosts conceded twice in a lacklustre first 10 minutes, with Kenedy and Yoshinori Muto breezing in to score.
United briefly roused themselves with Marcus Rashford heading wide from five yards, but also needed David de Gea's excellence to deny Muto his second and a potentially decisive third for the Magpies.
The hosts departed at the break to jeers - some of which were aimed at the club's owner and executives, rather than the manager.
But by the final whistle, it was the name of Mourinho that rang around Old Trafford.
- Analysis - two days bordering on the unbelievable for Mourinho
- I can cope with manhunt - Mourinho
- All the reaction from Old Trafford
- What else happened in the Premier League on Saturday?
The Portuguese, who brought Alexis Sanchez, Juan Mata and Marouane Fellaini off the bench in pursuit of victory, saw his side reenergised after the break, when two of his three substitutes found the net.
As Newcastle dropped deeper, Mata swept home a free-kick with 20 minutes left to trigger an onslaught on the visiting goal.
Martial curled home six minutes later from Paul Pogba's pass and, after goalkeeper Martin Dubravka pawed away Chris Smalling's effort, Sanchez delivered the winner in the last minute of normal time, heading in Ashley Young's cross.
Manchester United's calamitous start
According to one report on Saturday, the club hierarchy had decided before this game to get rid of Mourinho regardless of today's result.
BBC Sport's understanding is that Mourinho, in fact, still has the support of the club's board.
Their faith might have been tested by an astonishing opening.
As chief executive Ed Woodward watched on grim-faced in the stands, United's defence crumbled, conceding twice in the initial 10 minutes of a Premier League game for the first time.
First, a lunging Nemanja Matic failed to intercept in midfield before Young tamely allowed Kenedy to turn on to his favoured left foot and fire in.
Three minutes later, the hosts were either too uncertain or unwilling to close down Jonjo Shelvey, allowing him to sweep into the heart of the box, where Muto easily turned Young and buried Newcastle's second.
Mourinho, who had joked with former Chelsea charge Kenedy in the tunnel and playfully squirted a water bottle at a television camera before kick-off, appeared far less jovial by then.
As he gesticulated wildly on the sideline, his players looked at each other in apparent shock.
Mourinho finally finds solution
Mourinho acted quickly in response, sacrificing Eric Bailly, a £30m specialist signing, after just 19 minutes. He installed Scott McTominay, a relatively inexperienced midfielder, in the centre of defence in order to make space for Mata.
That reflected a season of confusing decisions and a summer of transfer frustration during which Mourinho made no secret of his desire to sign another centre-back.
Manchester United certainly looked no more solid after the switch as Muto came close to a third and Mohamed Diame strode within striking range unchallenged.
The home side were on to their third defensive formation by the start of the second half as Fellaini replaced McTominay. With Mourinho's half-time words apparently having the desired effect, they finally hit on a line-up that worked.
United were more purposeful and urgent, and chased down the game with a determination reminiscent of some of Sir Alex Ferguson's finest days.
The glee on the final whistle should be tempered by the strength of the opposition, though.
Newcastle are without a win in their eight league games this season and, once Mata's goal went in, lost belief and shape. Against tougher opponents, Manchester United may well have found such an escape impossible.
Newcastle shortcomings exposed
After meeting manager Rafael Benitez and some of the squad at an Italian restaurant on Wednesday in an apparent bridge-building exercise, Newcastle owner Mike Ashley was in attendance for his third match in a row.
The visitors are in dire need of his interest and, more importantly, his investment.
Two points from eight games represents the Magpies' worst start to a top-flight season in the era of three points for a win.
With only goal difference keeping them from the bottom of the table, Benitez will surely lobby for more money to spend in January after failing to persuade Ashley to back his judgement in the summer.
Man of the match - Juan Mata (Manchester United)
'We promised that we would give everything' - what they said
Manchester United manager Jose Mourinho: "Marcus Rashford was sad on the pitch, Scott McTominay was scared on the pitch. Even older players commit mistakes that are not normal.
"At half-time we had a good conversation. We didn't promise that we would win the match. We promised we would give absolutely everything without any fear, without any pressure, just the pressure of giving everything and of course they gave everything.
"We fought so much. They deserve even if the game ends 2-2. The feeling would be a positive feeling because nobody can promise to win matches."
Newcastle manager Rafael Benitez: "It is difficult to take because we did so well. At 2-0 we had chances to score the third and we had control against a very good team.
"When we conceded the first goal, we started to have problems because they have so many good players that they could bring off the bench, and it is difficult to contain them.
"We have to stay calm and be sure we continue working in this way, because we will win games if we play like that."
United's magnificent seventh comeback
- Manchester United registered their seventh Premier League win after being two or more goals down at half-time - four more than any other team.
- This is Newcastle's worst-ever start to a top-flight season after eight games in terms of points (2).
- Newcastle are winless in their eight Premier League games this season - only in 1898-99 (first 10) have they gone longer without a win from the start of a top-flight campaign.
- Newcastle have lost eight Premier League matches when scoring first against Manchester United.
- Newcastle found themselves leading for the first time this season in any competition.
- Since his Premier League debut in August 2011, Mata has scored more direct free-kick goals in the Premier League than any other player (8).
- Paul Pogba has had a hand in 29 Premier League goals under Mourinho at Manchester United (13 goals, 16 assists), more than any other player.
- Since 2016/17, Manchester United attacker Alexis Sanchez has scored three 90+ minute winners in the Premier League, more than any other player.
Once the international break is over, Manchester United travel to Chelsea on Saturday 20 October (12:30 BST), and Newcastle entertain Brighton on the same day (15:00).
- 1De Gea
- 3BaillySubstituted forMataat 19'minutes
- 23ShawBooked at 45mins
- 39McTominaySubstituted forFellainiat 45'minutes
- 6PogbaBooked at 63mins
- 10RashfordSubstituted forSánchezat 67'minutes
- 11RitchieBooked at 58mins
- 10DiaméBooked at 69mins
- 15KenedySubstituted forMurphyat 68'minutes
- 17PérezSubstituted forJoseluat 88'minutes
- 13MutoSubstituted forAtsuat 78'minutes
- Anthony Taylor
- Shots on Target
Second Half ends, Manchester United 3, Newcastle United 2.
Corner, Manchester United. Conceded by Federico Fernández.
Attempt saved. Alexis Sánchez (Manchester United) right footed shot from outside the box is saved in the bottom right corner. Assisted by Anthony Martial.
Foul by Alexis Sánchez (Manchester United).
DeAndre Yedlin (Newcastle United) wins a free kick in the defensive half.
Goal! Manchester United 3, Newcastle United 2. Alexis Sánchez (Manchester United) header from the centre of the box to the bottom left corner. Assisted by Ashley Young with a cross.
Substitution, Newcastle United. Joselu replaces Ayoze Pérez.
Offside, Manchester United. Paul Pogba tries a through ball, but Juan Mata is caught offside.
Attempt blocked. Paul Pogba (Manchester United) right footed shot from outside the box is blocked.
Corner, Newcastle United. Conceded by Nemanja Matic.
Corner, Newcastle United. Conceded by Chris Smalling.
Corner, Newcastle United. Conceded by Chris Smalling.
Substitution, Newcastle United. Christian Atsu replaces Yoshinori Muto.
Ashley Young (Manchester United) wins a free kick in the defensive half.
Foul by Matt Ritchie (Newcastle United).
Attempt missed. Mohamed Diamé (Newcastle United) header from the centre of the box is close, but misses to the right. Assisted by Matt Ritchie with a cross following a corner.
Corner, Newcastle United. Conceded by Ashley Young.
Goal! Manchester United 2, Newcastle United 2. Anthony Martial (Manchester United) right footed shot from the left side of the box to the bottom left corner. Assisted by Paul Pogba.
Attempt saved. Paul Pogba (Manchester United) right footed shot from the left side of the six yard box is saved in the centre of the goal. Assisted by Anthony Martial with a headed pass.