Haaland has now scored 67 Premier League goals since joining City in the summer of 2022.
Mainoo went on to play 32 times for United, scoring five goals.
The game ended 2-1 in favour of Manchester United.
In the 87th minute, substitute Antony sent the game into extra time.
Manchester United returned into the second half much stronger.
In other games, Manchester City have yet to win any of their last four league matches.
“It’s one of the best goals I’ve ever scored.’’
The Red Devils paid €50 million, which could rise to €60 million with add-ons to the Italian side, making him the third most expensive goalkeeper in history.
In his post announcing the news, the former Leicester City defender said he is “extremely disappointed” with the manager’s decision.
“I just wanted to send this farewell message to all Manchester United supporters,” the 32-year-old posted on Instagram on Saturday.
