You would think Manchester United had seen it all in 13 years of trouble and toil since Sir Alex Ferguson left the club, but this is a week that is going to give people at Old Trafford nightmares for years to come.

If you thought a 15-year-old demanding to leave the club was a blow, it was nothing compared to what happened on the pitch a day later. This is a defeat that Michael Carrick’s reign might never recover from, so damaging was it.

United hadn’t thrown away a two-goal lead at Old Trafford for 50 years, and this was the 146th time they had led 2-0 at Old Trafford since Ferguson called it a day. As bad as things have got since 2013, they had never plumbed this depth. On 143 occasions, they had gone on to win, and twice they had drawn.

Now they have lost, and the worst part is that for most of the 35 minutes this game was 2-0, you actually thought there was a good chance they would lose. They had a start Carrick would have dreamt of after the turmoil of Tuesday, but showed no control thereafter.

Carrick spent £155million on midfielders this summer to provide just that and while one of them was injured, his new-look pairing of Andrey Santos and Youri Tielemans were schooled by the smarter, calmer and more composed Brighton midfielders.

This was a disastrous night for Carrick. It’s on his watch that one of the few records United haven’t trashed in recent years has now fallen and it already feels like he might not be able to recover from this. A two-year contract was evidence that not everyone in a position of power at Old Trafford was convinced he was the right man for the long term.

After two defeats in 17 games last season, he has now lost three of six this term, and the fact that Fabian Hurzeler’s Brighton looked better coached and had more distinctive patterns of play reflects poorly on the United head coach. He badly needs a result at Fulham now.

That fast start felt like a distant memory as Old Trafford seethed with rage in the final 20 minutes, with United continually coughing up possession and looking out of ideas. Boos greeted the final whistle and the players faced whistles as they did a lap of the pitch at full-time.

Shea Lacey was perhaps the only one of them who could feel exempt from the criticism. This should have been his night and some of his shambolic teammates owe him an apology.

The 19-year-old had made a bright start to the game, carrying the ball aggressively up the right flank and then fizzing one direct ball into Benjamin Sesko, but the moment he delivered eight minutes in felt the perfect tonic for United.

It was a move becoming familiar to Lacey. In a certain way, it’s Arjen Robben-esque, the drop of the shoulder and the cut inside his man to make space for the shot. Jaouen Hadjam was the outfoxed full-back on this occasion and Lacey’s decision to go for the near post from 18 yards out caught out Jason Steele.

The presence of Lacey means scousers will play more minutes for Manchester United this season than they will for Liverpool and the sight of someone born down the East Lancs Road kissing the United badge at Old Trafford would have caused a social media backlash from those with an L postcode.

But after a build-up dominated by the talk of academy pathways and bright young talents, the fact that Lacey is on a fast track to stardom in Manchester rather than Liverpool is a success story for United, whatever comes next for Gabriel. It’s about their only success story at the moment.

Mason Mount doubled the lead after good work from Marcus Rashford, but the wheels were coming off well before Brighton finally scored.

They looked the more cohesive, more adventurous and more structured team, and they played through United at will. Malick Yalcouye gave Diogo Dalot a difficult time and the Seagulls should have scored before they did.

Ayden Heaven had already found the going tough, and he played Charalampos Kostoulas onside, appealing for a free-kick 40 yards away rather than dealing with the danger around him. But the greater error was Karl Darlow’s, the goalkeeper on debut, remaining rooted to the spot rather than coming to collect the long ball, allowing Kostoulas to finish through his legs from close range.

Carrick had seen enough of United’s lack of control and had a triple substitution ready when Brighton levelled. United’s attempts to play out from the back lacked any conviction and the move between Andrey Santos, Heaven and Leny Yoro only sent the ball further into danger. Dalot hacked clear, but when Luka Vuskovic beat Sesko to the header, Pascal Gross was in space between the centre-backs to find a calm finish.

United were clearly in panic mode now and it got worse for them within four minutes. A slick move down the right left Noussair Mazraoui chasing shadows and Dalot and Yoro were marking shadows when the low cross came in for Kostoulas. Maxim De Cuyper accepted the invite to put Brighton ahead.

Rashford had United’s best chance to equalise, only for Steele to save with his legs. Brighton looked more likely to make it four, cutting through United’s wretched defence with ease. They declared at three, but the damage had been done.