Erik ten Hag urged Manchester United to show some "character" after they plunged deeper into crisis as Brighton swept to a shock 3-1 win at Old Trafford on Saturday.
Hannibal Mejbri's first goal for the club was no consolation for United, who were booed off at the final whistle.
It was United's second successive defeat after their 3-1 loss at Arsenal before the international break.
With three defeats in five league games already this season -- the first time they have suffered that fate in the Premier League era -- United are languishing in 12th place and have the look of a club in turmoil on and off the pitch.
"Definitely that is something that bothers me. Finally it's about character. Now we have to see how strong we are, how the team sticks together and which players are standing up and showing the character and leading the team," Ten Hag said.
"Because in all the games, all the games but especially the games today, against Arsenal and Nottingham Forest, we have seen we can play very good and we can create a lot of chances.
"But, yes, there are also some improvements to make. That is definitely the case and now we have to step up."
Ten Hag appeared to have stabilised United last season when he ended the club's six-year trophy drought by winning the League Cup and qualifying for the Champions League in his first campaign.
But hopes of a title challenge are already fading after a dismal start to the new season, with a daunting Champions League opener at Bayern Munich on Wednesday threatening to add to United's woes.
"We have to be very disappointed," Ten Hag said. "And we have to be very annoyed with ourselves because at United the demand is you win games."
In response to a rash of disciplinary issues involving his players, Ten Hag claimed this week that he arrived at Old Trafford following his move from Ajax to find "no good culture" among the squad.
On Friday, Ten Hag said he did not know whether exiled forward Jadon Sancho will play for United again.
Sancho has been banished to train away from United's first team after the England international angered Ten Hag when he claimed he was a "scapegoat" after he was dropped for the Arsenal loss.
Meanwhile, United forward Antony is on a leave of absence to fight allegations of domestic violence made against him by three women, which he strongly denies.
- United torment -
Antony's problems came weeks after Mason Greenwood was forced to leave United on loan to Spanish side Getafe over domestic abuse allegations that were later dropped.
United fans have also continued their protests against the disliked Glazer family after the club's owners appeared to halt their sale plan.
Gross has made a habit of tormenting United in the past and the German made it seven goals in 11 games against them when he collected Tariq Lamptey's 53rd minute pass, danced past Lisandro Martinez and fired a clinical finish into the bottom corner.
Ten Hag was loudly jeered when he opted to replace Hojlund with Anthony Martial.
Bizarrely, Ten Hag said: "I think it was positive. You see that the fans from the first moment in Old Trafford, the reception for him was great."
In the 71st minute, Pedro produced a superb finish from Lamptey's pass, prompting United fans to trudge to the exits.
Hannibal came off the bench to curl home from 20 yards two minutes later, but it was too late to stop the crisis from engulfing United.