The Gunners went down 2-0 in their opening group match, and the Spaniard insists that improvement is needed if they are to compete at the highest level.
"If you make the mistakes that we did, against top sides, we know that there is no chance to win the Champions League," The Mirror quotes him as saying.
"We encouraged them to do that by giving the ball away so many times in very difficult positions. They exploit the spaces very well and they put us under pressure. I am bitterly disappointed. I am very disappointed because we knew their strengths, what we had to avoid and what we did was encourage them to play their game.
"We have to react. We have to analyse what we have done wrong and try to improve. In four days, we have another game. That is the best thing, another game to try to react and improve."