Juan Mata has voiced his frustration with Manchester United's display against Arsenal and feels their start at the Emirates Stadium was "unacceptable".
Louis van Gaal's men fell 3-0 behind within 20 minutes and failed to get back in the game in the remaining 70 minutes.
Mata believes United will struggle to compete for trophies if they do not improve on Sunday's performance, even if he feels they improved after half time.
"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," Mata wrote in his weekly blog.
"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 [Sanchez] 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."
