Manchester United boss Erik ten Hag was frustrated with his team's defending for their 3-2 defeat at Arsenal.
Marcus Rashford's opener - his ninth goal in all competitions since the World Cup - was the dream start Ten Hag would have hoped for, but the United boss was disappointed with his team's lack of concentration.
"All the goals we conceded today are mistakes - it can't happen," the Dutchman told Sky Sports.
"Even last week, we were so hard to beat in such situations. Today we were not. All three goals, we could have avoided."
While he conceded that the Gunners are "good on the ball", Ten Hag said his side are a "good defending team" and should have done better against the league leaders.
"There were not a lot of open chances," he added. "We made mistakes which we cannot make. This game was decided by small details but also the decision making.
"That is hard to accept and we have to learn from it. 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."