Manchester United's fragile defence was constantly exposed as they conceded four first half goals in a heavy and embarrassing home defeat to Tottenham.
After Bruno Fernandes gave United the lead from the spot in the second-minute, the hosts crumbled and found themselves behind just five minutes later.
Tanguy Ndombele was handed a gift to level the scores after Eric Bailly, Harry Maguire and Luke Shaw made a mess of their clearance, before Harry Kane released Heung-min Son whose cool finish gave Tottenham the lead in the seventh-minute.
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Things went from bad to worse for Ole Gunnar Solskjaer as Anthony Martial was shown a straight red for a slap on Erik Lamela, whose theatrical reaction gave referee Anthony Taylor no other choice.
And before the interval, United conceded twice more, with Kane registering on the score-sheet after Bailly gave up possession, and Son made it four with a clever near-post finish from Serge Aurier's cross.
Their misery continued six minutes after the restart when Aurier drilled home Tottenham's fifth, with Paul Pogba to blame for not tracking his runner. But Spurs weren't finished there, as Kane scored his brace from the penalty spot after a rash Pogba foul on Ben Davies.
The victory moves Spurs to fifth, as Jose Mourinho made an astonishing return to Old Trafford, while shambolic United fall to 16th with a goal difference of -6.
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After yet another frustrating transfer window for Manchester United, this was a wake-up call to the hierarchy if there was ever one. Donny van de Beek has been their sole major signing - and he has yet to make his first Premier League start.
The signings of veteran striker Edinson Cavani and left back Alex Telles are seemingly imminent, but United have been crying out for a new centre-back for quite a while - and this has never been more evident. Bailly - given the nod ahead of Victor Lindelof - is simply not good enough for a club at this level.
With just over 24 hours until the transfer window slams slut, United have left it too late - again - and that is a frightening reality for Solskjaer, who also must face stern questions over one of the most shambolic Manchester United displays in Premier League history.
This was the most embarrassing of defeats for United, but credit to Tottenham, who were magnificent. Considering they have played four games in seven days - drawing one and winning three - Spurs were much sharper, energetic and hungrier than their opponents. Mourinho's sides are known to be pragmatic, but Spurs have now scored 12 in four Premier League games and are only growing in confidence.
MAN OF THE MATCH - Serge Aurier (Tottenham)
Matt Doherty may struggle to win his place back after this impressive showing from the 27-year-old. Every Spurs player was a contender, but Aurier's dynamic performance deserves most credit.
Man Utd: De Gea 5, Wan-Bissaka 4, Bailly 3, Maguire 3, Shaw 3, Matic 4, Pogba 3, Greenwood 5, Fernandes 5, Rashford 5, Martial 3.. subs: Fred 5, McTominay 5, van de Beek 5.
Tottenham: Lloris 6, Aurier 9, Sanchez 6, Dier 7, Reguilon 7, Hojbjerg 7, Sissoko 6, Ndombele 7, Lamela 7, Son 8, Kane 8.. subs: Davies 6, Alli 5, Moura 6.
2' - GOAL! Manchester United 1-0 Tottenham (Fernandes, pen): Fernandes steps up and rolls it confidently past Lloris! There have been some questionable penalties awarded this season, but Jose Mourinho cannot complain about this one!
4 ' - GOAL! Manchester United 1-1 Tottenham (Ndombele): United make a right mess of it at the back, schoolboy stuff as Shaw and Maguire fail to clear, and Ndombele profits to peg the hosts back immediately!
7' - GOAL! Manchester United 1-2 Tottenham (Son): And another! Spurs are ahead! Kane plays through Son, who lifts it over the on-rushing De Gea and sees his shot nestle in the bottom corner! Wow, wow, wow!
29' - Red card! Manchester United are now down to 10 men! Martial is given his marching orders for a slap on Lamela!
31' - GOAL! Manchester United 1-3 Tottenham (Kane): Another horror show at the back from 10-man United! Bailly tries to play it out from the back, but his attempted ball to Matic is cut out by Kane's sliding tackle and he's punished! Son's cross is slotted it into the bottom corner by Kane! Shambolic!
37' - GOAL! Manchester United 1-4 Tottenham (Son): It's FOUR! This is embarrassing! Aurier receives Sissoko's diagonal ball and nutmegs Maguire with his cross, leaving Son with a simple tap in at the near post!
51' - GOAL! Manchester United 1-5 Tottenham (Aurier): Jose Mourinho must be loving this, as Aurier makes it FIVE! Shaw is dragged out of position and Pogba doesn't track the wing-back, and he's allowed to run onto Hojbjerg's pass to smash a low shot past De Gea.
79' - GOAL! Manchester United 1-6 Tottenham (Kane, pen): Kane buries the penalty into the corner, the ball carrying too much pace to give De Gea any chance at all.
- Manchester United have conceded six goals in a single Premier League game for only the third time - with each coming in October (1996 v Southampton, 2011 v Man City, today v Spurs).
- United are the seventh side in Premier League history to concede a penalty in three or more successive home games and the first since Arsenal in December 2016.
- Harry Kane has been directly involved in nine goals in the Premier League this season (three goals and six assists); only Thierry Henry in 2004/05 has had a hand in more in the opening four games of a campaign (10).
- Jose Mourinho is the first manager to face a side he's formerly managed in the Premier League away from home and score five goals since Ole Gunnar Solskjaer versus Cardiff in December 2018 (5-1).
- Tottenham are the first visiting side to score four first half goals in a league game away at Manchester United since Spurs themselves also did so in November 1957 in a 4-3 win.
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