These are desperate days for Man United: After derby defeat and academy star JJ Gabriel's public snub, it gets even worse for Michael Carrick as Brighton fight back to knock them out of the Carabao Cup

What desperate days these are for Manchester United.

Beaten in the Old Trafford derby on Sunday, at odds with their own fans and snubbed by the superstar of their academy, it felt as though things couldn't get any worse. Until this.

Two goals up against Brighton on Wednesday night, United were beaten at home for the second time in three days as they crashed out of the Carabao Cup at the first attempt for the second year in a row. At least Brighton aren't Grimsby.

It had all looked so promising when 19-year-old Shea Lacey celebrated his first start with a goal. It felt like it was meant to be following the shock news that his Under-21s team-mate JJ Gabriel - who could also have played in this game - announced he wanted to quit the club.

Mason Mount put United two up, but Brighton roared back to win for the fifth time in six visits to Old Trafford with goals from Charalampos Kostoulas, Pascal Gross and Maxim de Cuyper.

It was also the first in 146 games that United have lost at home after being two goals ahead.

After all the furore over Gabriel, the stage was set for Lacey to carry the torch for United's academy and he didn't disappoint.

Ironically, his first start came against Brighton who were United's opponents for Lacey's home debut at Old Trafford in January - in the last game before Michael Carrick took over from caretaker Darren Fletcher - which ended in despair for the youngster when he came on as a substitute and was swiftly sent off for two bookable offences.

Lacey has stayed at the front of the queue of academy players pushing for a first-team place since then, and Carrick rewarded him as he made six changes from the Manchester derby.

'We've always had hopes for Shea,' said the United boss before kick-off. 'Balanced hopes, you know, because you never want to get too carried away.

'We're not putting everything on him. It's not one chance, one game and that's it. But it's certainly a good opportunity for him.'

If Carrick was concerned about putting too much pressure on the boy, he need not have worried.

There was something inevitable about the fact that he fired United ahead. Noussair Mazraoui played the ball to Youri Tielemans, United's £35m summer signing who was captaining the team on his sixth appearance, and the Belgian picked out Lacey on the right side of the box.

He was marked closely by Jaouen Hadjam, but Lacey took one touch to control the ball and two more to get it out of his feet and wide of the Brighton defender before whipping a shot inside Jason Steele's near post.

Old Trafford rose to salute the first teenager to score on his United debut since Marcus Rashford against Midtjylland here a decade ago. Lacey kissed the badge and Carrick raised both arms to the fans behind the dugout in the Sir Bobby Charlton Stand.

They had only just settled down when Mount had them off their seats again in the 10th minute. Rashford picked Costinha's pocket on the other flank and breezed past Luka Vuskovic. Steele kept out his effort but the ball ran to Mount who got there just ahead of Lacey to score the second.

At that stage, it looked like being an easy night for United. Rashford looked particularly dangerous down the left where he had Costinha on the run and delighted the fans when he nutmegged Chema Andres.

But Brighton were not about to lie down after conceding two goals in two minutes and began to put pressure on a defence that had youngsters Leny Yoro and Ayden Heaven at the heart of it.

Heaven was caught out in the 27th minute by Matt O'Riley whose cross was cut out by Yoro, and Olivier Boscagli fired narrowly over the bar of United's debutant goalkeeper Karl Darlow.

Malick Yalcouye should have scored when he was sent clear by Gross, hotly pursued by Heaven, but placed his effort wide of Darlow's far post, and it needed a brilliant sliding tackle by Tielemans to deny Kostoulas.

When Kostoulas got Brighton back into the game in added time at the end of the first half, it was the least they deserved.

United were unhappy that referee Farai Hallam didn't give Benjamin Sesko a free-kick when he was floored by a tag team of Yalcouye and Boscagli, but they switched off and Hadjam's long diagonal pass picked out Kostoulas who had got away from Heaven. The Greek controlled the ball before knocking it past a hesitant Darlow.

It felt as though Carrick needed to make changes at half-time to alter the flow of the game but none were forthcoming.

Instead, Brighton continued to push after the restart and equalised in the 64th minute. Diogo Dalot kicked a clearance towards halfway where Sesko was under pressure from behind by Pascal Struijk and succeeded only in heading the ball 30 yards back towards his own goal where Gross was waiting to receive it on the edge of the box. He sold Yoro a dummy before sliding a shot past Darlow.

Bryan Mbeumo, Joshua Zirkzee and Kobbie Mainoo were waiting to come on at the time, but their introduction couldn't prevent Brighton shocking Old Trafford by going ahead.

Kostoulas got away from Mazraoui in the 69th minute and closed in on Darlow's goal. His cross got a deflection off the Moroccan and fell for substitute De Cuyper who slotted in the third to pile more misery on Carrick and his players.