United led 2-0 after just 10 minutes against Brighton on Wednesday night. By full-time, they had lost 3-2, exited the Carabao Cup at the third-round stage for a second successive season and been booed off by their own supporters.
It was the first time in 50 years United had lost at Old Trafford after going 2-0 up. They had won 143 of the previous 145 matches in which they had established that advantage. They also failed to register another shot on target after Mason Mount's 2-0 goal until the 76th minute.
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The defeat leaves Carrick under pressure only months after his interim spell was rewarded with the permanent job. He had won 12 of his 17 matches last season, drawing three and losing two, giving him a 70.6 per cent win rate. This season, his record is two wins, one draw and three defeats from six games - a 33.3 per cent win rate.
United have now also matched their joint-worst start to a Premier League season after four games, while their Carabao Cup exit has removed an opportunity for silverware.
So what is the main issue at Old Trafford?
United are being outrun
United's lack of urgency on the pitch has been stark, and the numbers match what we're seeing in matches.
United rank 14th in the Premier League for distance covered this season, with 449.1km, but their rankings drop sharply when the intensity of those movements is measured.
They are 18th for sprints, with 379, 19th for high-speed runs, with 1,364, 18th for sprints per kilometre covered and 19th for high-speed runs per kilometre.
That paints a picture of a team covering ground without doing enough of it at the speeds required to press, recover and defend transitions.
It was particularly obvious against Brighton. United's nine changes from the side that started the Manchester derby provided some context, with several players coming into the team after spending time on the bench. But Brighton also made six changes, including four players making their first starts for the club.
Jamie Redknapp questioned United's physical condition.
"They didn't look fit enough," he said. "I don't know if a lot of them do enough. I saw too many players not running back or not looking fit enough."
Leny Yoro's recovery for Brighton's third goal was one example. He was jogging back as Brighton broke, rather than sprinting into a position from which he could make a challenge.
It is a concern that stretches beyond one night. If United cannot match opponents for intensity, they are left defending larger spaces and spending longer without the ball.
Your views
Bomer86: "Why are Brighton fitter and faster than Man Utd? What is happening in training that the players are not fit? They were slower to every ball. No desire, no energy after 10 mins."
Ossy: "Fitness/intensity and Carrick's inability to read game state and react accordingly is going to be Man Utd's undoing this season."
United have spent, but is the squad balanced?
This summer United made seven permanent signings, alongside four loan arrivals, while 13 players left permanently and 10 went out on loan.
The headline investment has been concentrated heavily in midfield and attack. Around £85m of that spending started against Brighton in Youri Tielemans and Andrey Santos, while Carlos Baleba - signed from Brighton for around £70m - was watching from the stands.
The broader squad remains one of the most valuable in the Premier League. Transfermarkt values United's first-team squad at £791.2m, the fifth-highest figure among the clubs shown, behind Manchester City, Arsenal, Chelsea and Liverpool.
However, concerns remain over whether the distribution of that value has provided Carrick with enough tools to work with.
United's forwards account for £317.2m of the squad valuation and midfielders another £249.9m. The defensive group is valued at £193.8m, with the goalkeeper position accounting for the remainder.
The defence was already an obvious area of concern before the transfer window closed, yet United went into Wednesday's game with two inexperienced centre-backs in Yoro and Ayden Heaven and limited options at full-back.
Your views
EmmaRachael: "Why do Man Utd persist in buying quantity instead of real quality."
Cheese: "Appalling defence please invest in defensive reinforcements."
Gazza D: "United haven't backed Carrick. Desperately need two wing-backs of quality and that is evident in what we've seen with Dalot and Mazraoui."
Owners and INEOS: Who built this squad?
Supporters have repeatedly questioned the direction of the club, particularly after another transfer window in which significant money was spent but defensive reinforcements remained an issue.
United have now cycled through several managers since Sir Alex Ferguson's retirement, yet familiar complaints about the squad, recruitment and performances remain.
That is why Carrick's position can't be judged in isolation. The manager is responsible for the team on the pitch, but the squad he is selecting was built through decisions made at board and recruitment level.
Your views
S. Prime09: "United had a real chance to build on last season. The transfer window was so important. But we wouldn't spend over £70m. And all our top targets went some other team."
Ziggy: "The club has been lacking a clear plan since Fergie left. They don't know what they're doing."
Carrick: From interim hero to man under pressure
Carrick's interim spell last season was impressive enough to earn him the permanent job.
But it is the nature of the performances that has caused concern as much as the results.
United have struggled to control games and Carrick has faced questions over his in-game management and team selection. His nine changes against Brighton left him open to criticism when United's performance deteriorated after the early two-goal burst.
He accepted full responsibility for United's abject performance after the game.
"We can't accept that as a group. I take responsibility for it," Carrick told Sky Sports. "The performance towards the end of the second half, that's on me."
Your views
Jefft: "I'm so grateful for Carrick getting us into the top four but I said it then, the club should have said thanks but get in an elite coach who tactically has it."
Jon S: "At what point do you stop blaming managers and start blaming players? You can't keep swapping manager after manager after manager."
United ignored defensive warning signs
Wednesday's game exposed perhaps the clearest squad weakness. Yoro and Heaven formed an inexperienced centre-back pairing and Brighton repeatedly found space around them.
For the second goal, Luka Vuskovic won the header ahead of Benjamin Sesko, and the ball fell to Pascal Gross, who had space to finish. Heaven failed to anticipate the danger and Yoro could not prevent the shot.
For the third, Yoro was again caught behind the play as Brighton broke.
Full-back remains another concern, with Diogo Dalot and Noussair Mazraoui facing criticism for their positioning and distribution - and the depth behind the established defensive options remains questionable.
Your views
John S: "The gaps between Heaven and Yoro were too big, the full backs were beyond poor and the midfield looks average."
HN: "Why would you use an inexperienced back line in a must win game? Nothing against playing young players but pair them with experience."
No more cohesion in attack
Last season, United had a more settled attacking combination with Matheus Cunha on the left, Bryan Mbeumo through the middle and Amad Diallo on the right. Sesko remained an option introduced off the bench.
Rashford's return has altered that balance, with supporters questioning whether the change has affected the team's attacking rhythm.
The time without a shot on target against Brighton is a major concern: after the early burst, United could not consistently supply Sesko or create enough pressure to prevent Brighton from taking control.
Rashford did have a late chance to equalise but saw his near-post effort saved.
Your views
Dhanyaal: "Last season Cunha on left and Mbeumo on right and Sesko as number 9 was working. Why's he changed it and put Rashford on left?"
Platty7: "Rightly or wrongly the major change from last season is Rashford, he hasn't exactly set the world alight."
VOTE: What do you think the biggest issue is at Man Utd?
