Erik ten Hag has a brand of defiance that sometimes made him sound a man apart. An unexpected taste of glory empowered him. “If they don’t want me, I go somewhere else to win trophies,” he said after defeating Manchester City in the FA Cup final. Manchester United initially decided – albeit after speaking to a host of other managers – that they still did want him. Now, four months on, comes the decision they do not.

The FA Cup final was the extraordinary anomaly, the silverware in a slump that has lasted 14 months. Ten Hag can leave Old Trafford arguing he ended United’s six-year wait for honours, that only Pep Guardiola won more during his time in England. True as that is, his reign dragged them to historic lows. Ten Hag, who shaped up during his debut season as United’s finest manager since Sir Alex Ferguson, instead became another who was chewed up and spat out. A reputation initially enhanced instead became diminished with every pratfall. It became ever harder to believe he steered Ajax to the Champions League semi-finals.

Since then, he has overseen United’s worst ever Champions League campaign. Their record currently stands at one win in their last 11 European games; they could not even defeat Ten Hag’s former club Twente. Last season was their poorest in the Premier League: with a lowest finish of eighth, their most defeats and the ignominy of a negative goal difference. Now they have made their worst ever start to a Premier League campaign.

While a series of supposed inferiors – Brighton, Bournemouth, Crystal Palace, Fulham – won at Old Trafford, sometimes emphatically, Ten Hag tended to fail the toughest tests. United’s away record against the best was wretched: one win from 15 attempts against the big eight, seven points from a possible 45. If a 7-0 defeat at Anfield was a nadir in one respect, it at least came in a season when much else went right.

Since then, much else has gone wrong. If, at a club that has been dysfunctional in many a respect, it is not all Ten Hag’s fault, and when there has been a backdrop of instability amid a change in ownership, it became ever harder to escape the sense many of the problems stemmed from him.

Even injuries, last season’s ever-present excuse, could not explain their failings, as this season’s setbacks, with far fewer absentees, illustrated. The list of teams who looked better coached than United seemed to grow with every opponent they faced.

If United’s previous Dutch manager, Louis van Gaal, brought sterility, Ten Hag instead offered a bizarre brand of chaos. His side conceded at least 20 shots in a game and three or more goals in a game with remarkable regularity. And yet they were also rarely prolific, the third lowest scorers now, outscored by the rest of the top half last season, failing to register 60 league goals in either full campaign.

United could be short of firepower in attack, disorganised in defence and, at times, seemingly without a midfield at all. Ten Hag’s “doughnut formation” – with a hole in the middle – was most apparent last season, when the only thing Casemiro did quickly was decline, but it pointed to structural issues with confusing tactics.

Casemiro was both a talisman of Ten Hag’s initial improvement and then an emblem of a subsequent slump. And if his arrival dates back to a summer when United pursued Frenkie de Jong in vain and instead spent £63m on a 30-year-old, it highlighted several themes of Ten Hag’s reign.

There was the colossal outlay – over £600m by the end, around £200m every summer – with few recruits that could be called genuine successes (the best parts of his legacy may be Kobbie Mainoo and Alejandro Garnacho, two Ten Hag did not buy). The Dutchman nevertheless always seemed to want more players, claiming that his side were a work in progress even when they appeared to be regressing.

If the inflated prices, particularly those paid by former football director John Murtough, were not entirely down to Ten Hag, the recruitment policy had Ten Hag’s imprint. He seemed to overrate Dutch football, conducting an experiment on how a side of Eredivisie all-stars could do in England and Europe.

Most damningly, impossible to escape, there was the £86m disaster of Antony, pound for pound the worst signing in United’s history, a woeful footballer with a weird price. Many of United’s problems with PSR could have been solved if they had just never signed Antony.

If United gave Ten Hag too much money, they invested too much faith in him, too: first Murtough and former chief executive Richard Arnold but after them, Sir Jim Ratcliffe’s regime which sacrificed another season by keeping Ten Hag and letting him spend again.

Go back to the winter of 2022-23, when Ten Hag had recovered from the false start of a debut loss to Brighton and a 4-0 thrashing at Brentford to go on a 40-game run that brought only four defeats and yielded the Carabao Cup. He was forging a reputation as the iron manager of Old Trafford, a decisive figure who was right. He had won his power struggle with Cristiano Ronaldo, dropped the captain Harry Maguire, got Marcus Rashford scoring.

When he took the United job, friends in the Netherlands worried that he lacked the charisma required. When he seemed strong and correct, however, he earned respect without needing to be exciting. Yet the longer his reign continued, the less he displayed a sure touch in his decision-making.

Some of his selections were odd: he kept Rashford in the team when he didn’t score, then rested him when he did. No manager seemed to substitute centre-backs quite as often. Even last season’s FA Cup quarter-final comeback against Liverpool was illogical, with Antony ending the game at left-back and Bruno Fernandes in the middle of the defence.

Clarity of thought gave way to confusion. Ten Hag had a strange kind of reasoning. Players – Maguire, Scott McTominay, Christian Eriksen, Raphael Varane – veered in and out of favour with no great reason. There were times when his rescuers in games were men such as Maguire and McTominay whom he had tried to sell. The Jadon Sancho saga was pointlessly damaging and if the fault was not all Ten Hag’s, nor was he blameless. He talked about standards, with precious little evidence they were improving.

And if everything can be overblown at United, there were so many embarrassments, so many defeats that were so damning. In the glow of the FA Cup final win, Ten Hag said he walked into a mess at United. Now he leaves one, too.