Erik ten Hag blamed defensive mistakes as his Manchester United side went down 3-2 at Premier League leaders Arsenal on Sunday to leave them 11 points off the top of the table.
A run of five successive wins had prompted title talk at Old Trafford but a draw in midweek at Crystal Palace and Sunday's defeat has halted United's charge.
United led early through a Marcus Rashford goal and after falling 2-1 behind drew level through Lisandro Martinez.
But Eddie Nketiah's 90th-minute close-range effort sealed a deserved win for Arsenal who dominated the second half.
"It was a great game with a lot of tempo and the audience is the big winner," Ten Hag, whose side are in fourth place behind Newcastle United on goal difference, told the BBC.
"Two top teams but of course we are disappointed when you lose this game at a late stage. All the goals we conceded today are mistakes -- it can't happen."
United were guilty of allowing Nketiah a free header after needlessly conceding a corner and then stood off Bukayo Saka as he fired Arsenal ahead early in the second half.
They rode their luck late on but Nketiah flicked home from in front of goal in the 90th minute.
"Even last week, we were so hard to beat in such situations. Today we were not. All three goals, we could have avoided," Dutchman Ten Hag said. "They are very good on the ball but we are a good defending team.
"There were not a lot of open chances. We made mistakes which we cannot make. This game was decided by small details but also the decision making. That is hard to accept.
"I want to win. It is not good enough. We want to be the number one but not if we make mistakes. That is clear and what I told them."
