The Portuguese, for all his bravado, departed with one of the worst win percentages of any Red Devils manager in history.
Odd then, that he would be relieved of his duties just when it was looking as if United might be turning a corner.
A 1-1 draw against Leeds, whilst disappointing, had kept the Red Devils in the Premier League's top six, but it was the venom in his outburst after the game that ultimately decided his fate.
Darren Fletcher, a club stalwart, was brought in to steady the ship, though another draw - United's fourth unbeaten game in a row - this time against relegation-threatened Burnley, wasn't in the script.
Fletcher hoped to stem the tide of disappointment
In their last cup game, United were dumped out of the Carabao Cup by minnows Grimsby; therefore, a third-round FA Cup tie against Brighton was not only the chance to right that wrong, but could also mean some respite from their league travails.
The Seagulls, however, represented a tough opponent, even on the back of a run that had seen them win only one of their last eight in the Premier League.
Despite United having beaten them in October with a comprehensive 4-2 victory, that was only the North-West giant's second win in the previous six H2H encounters, with all four of the others being lost.
In terms of their current form, the hosts had scored 10 and conceded 10 in the last six matches, which had seen just one victory, four draws and one loss.
Fletcher was undoubtedly going to look to the leadership skills and all-round excellence of captain Bruno Fernandes given that, since the start of January 2023, no player has been involved in more goals in the FA Cup than the Portuguese (13 - eight goals, five assists).
Fabian Hurzeler, meanwhile, had United's kryptonite in Pascal Gross, as the 34-year-old had scored more goals for Brighton against the Red Devils (seven, four of them at Old Trafford) than against any other side.
More defensive incompetence from United
A lively start from both sides had everyone in the Theatre of Dreams stands interested from the first whistle.
Diogo Dalot went close, and Bruno had also seen an effort off-target before Brighton had even had a sniff of a chance.
However, the visitors quickly turned the tables as Diego Gomez, Jack Hinshelwood and Georginio Rutter began to chip away at United's defensive incompetence.
Sure enough, after just 11 minutes, the Seagulls were ahead thanks to Brajan Gruda's first goal in all competitions since August against Manchester City in the Premier League.
A fourth time in the last five away games against United that Brighton had opened the scoring should've sounded the alarm, given the visitors had won the previous three they'd gone 1-0 ahead in.
Ominously for Fletcher and his under-pressure outfit, the hosts had only come back to win one of the nine previous games they conceded the first goal in this season (D3 L5), with that coming in a 2-1 win at Crystal Palace in November.
Another brief attacking spurt from the Red Devils, which saw Benjamin Sesko get an effort on target and Matheus Cunha twice try his luck, was all that could be mustered with 40.6% of the action being in the United defensive third.
Mainoo a shining light
The recalled Kobbie Mainoo was, at least, putting in a shift, and the three chances he helped to create were already more than any other player. In fact, it was the most by a Man Utd player in the opening 30 minutes of a match this season (all competitions).
A definite shift in the game came with the hosts' collective 72% possession of the ball. At the heart was Mainoo with a 95.2% showing, though Manuel Ugarte's 88.5% and consistent passes into the final third were also notable.
By the end of the half, there was only one team in it, with United winning 66% of their duels against Brighton (31/47); their highest percentage in a half of football at Old Trafford this season.
Although Sesko had touched the ball only nine times before the break, he more than made up for it in the second half.
The confidence that goals have brought to his play has been obvious, and the striker's five shots (three on target) by the end of the game were easily the most from a Red Devils perspective.
Welbeck back to haunt his former club
Unfortunately, as has all too often happened this season, the rot soon set back in.
As good as Fletcher's side had been leading into the break, they were appalling directly after it, and even before Danny Welbeck smashed home a glorious second for the Seagulls just after the hour mark, the visitors had already had four attempts either blocked or off target.
Welbeck's eighth against his former club is his joint-most against an opponent in his club career (also eight against West Ham).
The industry of both Gruda and Gross was a real feature of the latter stages, by which point they'd contested 20 and 13 duels respectively, and won at least half in both cases.
Both managers looked to freshen things up with multiple substitutions, and with five minutes to play, Sesko got the goal his play deserved.
A third in his last two United appearances was more than across his first 17 games for the club (two), though it was too little too late.
United's frustrations could be summed up shortly after, when Shea Lacey, making his first appearance for the club in the competition, was sent off for dissent just two minutes after being yellow-carded for a reckless challenge, thus making him the first teenager to be sent off for the club in all competitions since Luke Shaw against West Ham in February 2015.
Back to the drawing board for United, whilst the Seagulls fly on...
