When Manchester United face Newcastle in the Carabao Cup on Wednesday night, it will bring back memories of the highlight of Erik ten Hag’s time in charge at Old Trafford.

United’s triumph over Eddie Howe’s side in the final at Wembley in February ended a six-year trophy drought and rewarded the club’s decision to appoint Ten Hag as manager.

Just a week later, though, the Dutchman went from his highest point to his lowest as United were humiliated 7-0 at Liverpool.

It has been up and down since then, and Sunday’s abject surrender to Manchester City, as they lost 3-0 in the derby at Old Trafford, was another dark day.

Ten Hag’s tactical decisions were criticised with former United captain Roy Keane claiming the manager’s ‘head is spinning’ as problems pile up.

Erik ten Hag is under scrutiny after United produced another torrid performance in Sunday’s Manchester derby defeat

Ten Hag’s tactical decisions were criticised as United failed to show signs of improvement

Sir Jim Ratcliffe will be keeping a close eye on developments as he prepares to take over the running of football operations in return for a £1.4billion investment.

Another former Old Trafford favourite, Gary Neville, joked that United need Daniel Radcliffe not Jim Ratcliffe and claimed the state of flux is dragging his old club down.

‘I believe the toxicity that exists at the club eats alive every manager and player that comes here,’ said Neville.

Off-field issues surrounding Mason Greenwood, Jadon Sancho and Antony have also been blamed for United’s poor performances on the pitch, as has a lengthy injury list.

But as Ten Hag comes under more scrutiny than ever, Mail Sport explores some of the key questions being asked of him.

Why leave Varane and Reguilon on the bench against City?

United’s defence has been worst hit by injuries this season, with Luke Shaw and Lisandro Martinez sorely missed. It made Ten Hag’s decision to leave out Raphael Varane, arguably his best and most experienced defender available, and reprise Leicester City’s central defensive partnership of 2018-19, by playing Harry Maguire with Jonny Evans, surprising.

Ten Hag’s reasoning was he wanted a natural right-footer on the right, and vice versa on the left, against City’s press. But this doesn’t explain why Varane could not have partnered Evans, or why right-footed Victor Lindelof played at left back instead of left-footed Sergio Reguilon for 73 minutes.

Raphael Varane was left on the bench by Ten Hag for the derby encounter for tactical reasons

Why did United change their central midfield so early?

Ten Hag was criticised for bringing on Mason Mount for the more defensive-minded Sofyan Amrabat at half-time when United were still in the game at 1-0.

Mount went to No 10 and Christian Eriksen dropped into a pivot alongside Scott McTominay. However, neither Eriksen nor McTominay could give United’s defence the protection it needed against a team like City in the absence of the injured Casemiro, and Ten Hag’s side were overrun.

United’s problems in the middle have raised questions over Ten Hag making Casemiro and Eriksen two of his first signings after both players had turned 30.

United changed their midfield in the second half with Scott McTominay being tasked with giving his defence protection

What is Mount’s role?

From the moment United paid Chelsea £60million to sign Mount in the summer, questions have been asked about where he will play in Ten Hag’s side and there are still no clear answers. He has been on the bench for the last three games and was anonymous after coming on for the second half of the derby.

At one point as City attacked again, Mount looked left and right to his team-mates and shrugged his shoulders mid-run as if to say: ‘I don’t know who I’m meant to be marking!’

From the moment United bought Mason Mount, questions have been asked over his best position

What was the plan against City?

Ten Hag raised eyebrows on Sunday by claiming ‘the game plan went how we wanted’ and that United had fought ‘toe-to-toe’ with City up until the moment they conceded a soft penalty.

Most observers saw it differently as Pep Guardiola’s side dominated again, with former United defender Jaap Stam telling MUTV: ‘It was hard to watch this game. City dominated everywhere on the pitch. We didn’t see any aggression in this game. If you talk to the players or the coaches or the manager, they will say we wanted it too, but from the stands it didn’t look like they really wanted to go for it.’

United were easily beaten and outsmarted by their cross-city rivals in Sunday’s derby

After spending £400m on transfers, what is United’s best XI?

Ten Hag says he has never played his best team, but does he actually know what it is? The Dutchman has been in situ for 18 months and there is still no clear picture of how United want to play.

Ten Hag was appointed based on his track record at Ajax, so his assertion that United will never play the same kind of football as his old club may set alarm bells ringing, not least because he has signed so many players with experience of the Dutch league.

So, if not Ajax, what are United meant to be? ‘We’re now into his second season and none of us can explain what Manchester United try to do in terms of how they play,’ Jamie Carragher told Sky Sports. ‘They are playing underdog football and they have done since he came in.’

Is Fernandes the best choice for captain?

Former United skipper Keane led renewed calls for Fernandes to be stripped of the armband after he put in another petulant display that brought back memories of his antics in the 7-0 defeat at Anfield. The Portugal star continues to be one of Ten Hag’s better performers, but doubts persist over his temperament and suitability to lead the team.

Fernandes cut a frustrated figure on Sunday when he played wide on the right for the fourth Manchester derby to try to blunt City, while also accommodating Eriksen and then Mount at No 10.

Questions have been asked over whether Bruno Fernandes should be Man United captain

How do you get Rashford firing again?

Nobody epitomises United’s drop-off this season better than Marcus Rashford, who has scored just once in 13 games after finishing Ten Hag’s first campaign as the 30-goal top-scorer and subsequently signing a new £300,000-a-week contract.

Rashford’s body language once again belies deeper issues, as it did at his lowest ebb two years ago. He offered very little in the derby and indications that he held a private birthday party at top Manchester nightspot Chinawhite after Sunday’s game weren’t the best look, either.

Nobody epitomises United’s drop-off this season better than Marcus Rashford

Does Sancho have a future at United?

As United struggle for creativity and goals, it seems ridiculous to have had a £73m winger training on his own at the academy for the last two months.

Sancho will not apologise for effectively branding his manager a liar in public, and Ten Hag won’t have the player back until he does. The manager has been commended for the discipline he has introduced at United, but this saga has been allowed to drag on and is benefitting neither party.

The saga surrounding Jadon Sancho’s future is benefitting neither the player or United

What constitutes success this season?

Ten Hag marginally over-achieved in his first season by winning the Carabao Cup and finishing third in the Premier League, but it has set the bar higher for his second campaign.

Although the 53-year-old insists United are going ‘in the right direction’, they are eighth in the league and face a battle to progress in the Champions League.

As they prepare to face Newcastle in the cup again, Ten Hag desperately needs some consistency to get back on track.