United suffered their biggest defeat to Chelsea for 17 years and Mata has revealed how stunned the dressing room was afterwards.
“There’s a sense of pain and disappointment obviously,” he told Sky Sports. “It’s a painful result. We came here with the mission of winning and having a good game and from the first minute we started very bad.
“I felt when we were close to score our first goal they scored third goal and that was key. We were close to their box trying to create something and they score the third goal.
“In the first half they started better than us and that was key.”