The Blues midfielder has admitted that Rafael Benitez's men did not do enough to earn victory during Sunday's 2-0 defeat to Manchester City at the Etihad Stadium.
The Blues dropped down to fourth after their 2-0 loss to the reigning Premier League champion, and face a battle for Champions League qualification with both Tottenham and Arsenal finding form.
The Spaniard has described the west London outfit's showing at the Etihad Stadium as not good enough.
"The most important games are when you play against the best teams in the Premier League and you have to show you are at this level," Mata told Chelsea TV. "On Sunday it wasn't enough, the job that we did, and what we have to do now is just carry on, the season is not finished yet.
"When you play teams like Man City you have to perform at your best. We tried but in the first 25 minutes they were better at pressing us. At the end of the first half we changed the game a little bit and after we had a few chances, and in the second half it was an important moment with the penalty and Joe Hart made a very good save."