Juan Mata says Manchester United made an "unacceptable" start to their 3-0 defeat at Arsenal.
The Spaniard declared that "everything went wrong" in the opening 45 minutes and warned United have to learn lessons quickly if they are to become champions.
Arsenal were two goals ahead after seven minutes, before Alexis Sanchez added a third 12 minutes later, and Mata felt United left themselves with a virtually impossible task.
United started the weekend as Premier League leaders but, after winning their previous four games in all competitions, were soon in trouble at the Emirates Stadium.
"Everything went wrong in the first half," the Spaniard wrote on his blog. "Before the game, our goal was to keep the top spot of the Premier League, but it wasn't our day as we saw from the kick off.
"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. From that moment we tried to stop them but we didn't create chances, only a good one from Anthony [Martial] right before half-time.
"If we had scored that one, things could have been different, but nothing happened after the break. We tried to get back into the game, fighting until the end, but obviously we were penalised by our horrible start."