Writing for his personal blog, Mata said: "Today, I believe there's no need to start by describing how frustrated we feel in the team after losing at Watford.
"To be honest, after the 1-1 we really thought we were going to win the game. We had scored and we were playing near Watford's box.
"However, it was precisely in that good moment that they scored again and since then until the final whistle they were in better control of the situation, and we lost the three points. It was an unexpected disappointment."