United out of both cup competitions at the earliest point for the first time in 44 years, falling to both Grimsby Town and Brighton as the club prepare for another trophyless campaign.
Former United striker Danny Welbeck scored a second to seal the win for the Seagulls in what was another reminder for the Red Devils of the players who leave the club and often send a message to their old side.
It was the 35-year-old's eighth goal against United and his sixth visiting the stadium he used to call home and he joked after the game that drug testers may come for him after such a performance.
"I can't give away my secret. If I say too much, drug testers will come for me!"
"I'm good, I'm clean, I've got good genetics, good family," he added. "My mum and dad blessed me with good genes. A lot of hard work. I work hard every day, I look after myself. I eat well, prepare well for every single training session. I'm feeling good, feeling strong."
Brighton had never beaten United before in this competition and Welbeck, who spent 13 years at his hometown club before joining Arsenal in 2014, spoke more on interim manager Darren Fletcher who says played a key role in his successful career.
"He helped me massively as a young player at Man Utd.
"It was nice to see him on the sidelines. I have the utmost respect for Darren Fletcher."
Brighton will find out their next opponents on Monday night as they prepare for the fourth round where they may face the likes of Arsenal, Manchester City and Chelsea.