Mata said: "The most important games are when you play against Man United, Man City, Arsenal, Tottenham and Liverpool - the best teams in the Premier League and you have to show that you are in this level.
"The job that we did (against City) wasn't enough. We didn't score, we didn't get the three points, always when you play against teams like Man City you have to perform at your best."
Chelsea are still in the FA Cup and Europa League, but Mata insists securing Champions League qualification must come first.
He added: "It is our main objective now. Obviously we want to win the FA Cup and Europa League, we are still alive in these competitions, but the main aim is to be third and to play in the Champions League next season."