Across four days Manchester United have lost the 199th derby to Manchester City, were told by JJ Gabriel he wants to leave, and have now been knocked out of the Carabao Cup by Brighton.

Capitulating after being 2-0 up may be the pick of these dizzying blows, and when Marcus Rashford skated in with the chance to take the tie to 3-3 late on, there was no surprise he hit Jason Steele rather than the net.

Towards the end, Matheus Cunha was thrown on to join Bruno Fernandes, also a late substitute, illustrating how desperate Michael Carrick was. Yet Charalampos Kostoulas went close to scoring a second for Brighton, as United could not find a route to cancelling out Maxim De Cuyper’s 69th-minute strike that proved the winner.

After taking only four points from twelve in the league, serious questions will now be asked about where Carrick’s team are heading; they have to beat Fulham on Sunday to start correcting a dismal start to the season.

Carrick stated Sunday’s injustice against City would give his side an edge and Shea Lacey provided a cutting one when decorating a full debut with the sweetest of goals.

The 19-year-old shimmied in Brighton’s area, beating Jaouen Hadjam before firing beyond a helpless Jason Steele. As impressive here was Benjamin Sesko’s hold-up work in the buildup as this allowed red shirts to flood forward, Youri Tielemans feeding Lacey.

This was seven minutes in and less than 120 seconds later Mason Mount doubled United’s advantage.

Rashford’s pace and drive took him clear along the left, again inside the area. A trailing Steele boot saved the No 9’s effort but the ball bounced into Mount’s path and he drove home, via the turf, for a first goal since a win at Wolves last December.

Lacey’s strike drew a Carrick grin and delight from Mount, and you wondered how Gabriel – earmarked for this game’s squad, with the chance to become United’s youngest ever player – felt.

Brighton had lost only one of their last five visits here, with a flying start to the league campaign placing Fabian Hürzeler’s team in fifth. Yet Sunday’s 5-0 hammering of Coventry already seemed distant as United continued to dominate.

Olivier Boscagli hit a corner straight out, summing up the visitors’ start, before Matt O’Riley forced Ayden Heaven into an error that led to another. This presaged a Seagulls renaissance that featured a lightning Malick Yalcouyé break but, clear in on the debutant Karl Darlow’s goal, the Ivorian spooned wide. The No 35 lacked the sharpness evident in a Rashford nutmeg of Chema Andrés which embarrassed the Brighton man and when Pascal Gross blasted over this was one more chance spurned by Hürzeler’s men.

But then came a goal – and real hope – for Brighton. Hadjam’s long left-right diagonal cut United’s rearguard clean out, and the slumbering Heaven allowed Kostoulas to scamper free, the striker beating Darlow through his legs.

Carrick, having beaten only Ipswich so far this season, needed a win, and as United (not under him) exited at the earliest time of both cup competitions last year, progressing to at least a second tie in one of them would be welcome.

A wild Tielemans long-range shot that ballooned over failed to restore United’s two-goal lead but at least the home unit were tapping the ball about, operating in Brighton territory, moving the guests around.

A Sesko burst from halfway had the onrushing Steele hacking away just in time so United were in control. But the question was: could they maintain this and register the type of mature, accomplished display that would show an evolution in Carrick’s team? Sesko, in yards of space, dawdling and dispossessed when a simple ball would have fed Lacey, suggested not.

A loose Rashford pass aimed for Sesko – the same. More promising was a pass for Noussair Mazraoui as he drove into the area along the left channel. As was the subsequent corner that created an opening for Tielemans to narrowly miss.

But then disaster. Diogo Dalot’s hoofed clearance failed to find Sesko as Luka Vuskovic headed to Gross; Brighton’s captain made room and netted with ease. Even worse followed. Brighton raided along the right, Kostoulas’s cross pinged off Mazraoui’s backside, and there was De Cuyper to finish.

On 72 minutes Carrick played his ace, introducing Fernandes for Mount, then followed up with Cunha. But United remained toothless.