And, damningly for the manager, two of his most high-profile recruits were at the scene.
"But after what's happened in the summer, if performances like that continue, I don't think Jose Mourinho will be the manager of Manchester United next season - he may not be manager at some time this season".
An evidently fuming Jose Mourinho only had one explanation for the surprise defeat, and that was the number of mistakes his young defence made in the first period.
Mourinho can't be exclusively blamed for his team's bad display against Brighton, but his negativity is clearly affecting his players. Something does not seem right - with him and his team - although those Brighton fans' prediction was never going to come true on Monday morning.
Victor Lindelof played well for Sweden at the World Cup but has not looked commanding since arriving at Old Trafford from Benfica a year ago. The Ivory Coast International is arguably United's most talented central defender, with speed, technique, passing range and bravery in the challenge.
To add to Young's misery, Juan Mata had a awful day at the office.
Despite this, Mourinho was careful not to name any individual players.
Harry Kane has this warning for his fellow stars
Tottenham have confirmed an agreement has been reached with UEFA to play their first Champions League group home game at Wembley. He has since been heavily involved in Chelsea's opening fixtures of the season, making starts against Huddersfield and Arsenal.
Mata lacks physicality and pace which is required at that position.
Do you want Jose Mourinho out? In the first half it was a hit.
The pressure on Mourinho is mounting following a disruptive summer that saw him fall out with Pogba and fail to sign some of his main transfer targets.
"Irrespective of the manager of Manchester United, they need new centre-backs". He was too slow to react during set-pieces and his cavalier attitude led to Brighton's third goal from the penalty spot.
Lindelof struggled through his first season in England but started this campaign with a strong performance against Leicester City. He isn't the most physical of defenders but to allow a 34-year old Glenn Murray harry you does raise a few doubts.
"He spoke about not getting the centre-halves", he told Match of the Day 2. United, though they lost to the same team at the end of last season, looked unprepared for the intensity of their opponents; surely they are unlikely to underestimate Spurs in the same manner. They are United in name only.
Manchester United signed Brazilian global Fred from Shaktar Donetsk for a fee believed to be in the region of £52m in June, and the 25-year-old was expected to provide steel and guile to the United midfield, allowing Paul Pogba the freedom to play to the best of his abilities.