The Spaniard spent three successful years at Stamford Bridge, before things went sour under Jose Mourinho, and he was sold to United in January 2014.
Asked in a fan questionnaire to name his best and worst moments in his long career, the 31-year-old replied: "The best one has to be the World Cup that we won in 2010 with Spain.
"That final when Andrés Iniesta scored the goal we were all checking the time, checking the referee, checking whether he was offside or not and then when the referee blew at the end of the game it was an explosion of happiness and energy and we had made history then, because Spain didn't win a World Cup before that time, so I think that is what every player dreams about, winning a World Cup for the first time with your country, and so that was the highest probably. Of course the Champions League, the Europa League [too] and I have been lucky enough, but the World Cup stands out."
Mata added: "And then the worst was probably when I was at Chelsea. I was voted, for two years, Player of the Year, I was playing regularly and I was playing very good and enjoying my football and then the situation changed.
"I stopped playing as much, my confidence was not as high and that was a challenge in my career that I had to overcome, but I think that it's normal. I don't know any player that has a career that is going always good and always right. There are injuries; there are managers that don't play football that is perfect for your qualities, there are moments in a club that are not going very well, so you have to adapt to these challenges and overcome them and then you will be an even better player."