Harry Maguire admits Manchester United were not good enough in Sunday's loss to Arsenal.
The Red Devils looked the inferior side throughout as a penalty from Pierre-Emerick Aubameyang saw Arsenal clinch their first win at Old Trafford in 14 years.
"It is hard to point a finger on it. We didn't start the game well, gave the ball away far too much in the first half," Maguire told Sky Sports.
"We can't find excuses so we just have to look within. We were not at it from the start.
"I don't think the concentration was there in the first half, in terms of the cleanness of the passing. Maybe we were a bit complacent [after the Leipzig win], everybody was patting us on the back.
"But you win and you lose, and you have got to stay level-headed, it is a long old season. I can't put my finger on it. We will analyse it this week. We have another big game on Wednesday [in Istanbul] and we have to move on quickly and we have to improve.
"It was a cagey game. A game of few chances. They had probably one or two chances, a bit more than us. We needed to do more, we needed to create more.
"It was important to start the game well and we didn't start the game well enough.
"We just gave the ball away too much. If you play any Premier League game and you give the ball away as much as we did, especially in that first half, you are not going to win games of football.
"It is not good enough. We have to work hard tomorrow in training and improve."
