Disappointed: Mata admits United were not good enough at the Emirates

The Gunners raced into a two-goal lead inside seven minutes to stun United, with goals from Alexis Sanchez and Mesut Ozil.

Sanchez made it 3-0 before the 20 minute mark, effectively taking the game beyond Louis van Gaal's men.

It brought to an end United's four-game winning run and Mata could not hide his disappoinment.

"As you could see, everything went wrong in the first half," the Spaniard wrote in his weekly blog.

"Arsenal started with more intensity, they scored very quickly and with no time for our reaction they scored the second goal. Not even 10 minutes have passed and things were already looking very bad.

"A start like this from us is unacceptable; if we want to fight for big things we must learn the lesson as soon as possible.

"The only thing I can say is that we shouldn't play a game like this again. When we were starting to react and move the ball, Alexis scored the third one.

"It's not easy to beat Arsenal at their stadium, but when you lose 3-0 after the first 20 minutes then it's almost impossible. We were penalised by our horrible start."

Mata, who will now join up with the Spain squad, insists United must bounce back starting with Everton after the international break.

"We have an important month of October ahead, with games that will demand our best level of performance, and we have to be ready for them," Mata added.

"We still are in the top spots of the table and this is a long-distance race, we are sure of that.

"All the teams are finding difficulties, but we remain hopeful that we can achieve something important this season, and that's what we are working for every week.