The manager said his team's displays on the road are a worry, insisting the squad "deserved to lose" to Newcastle as its six-game winning streak came to an end.
The Blues have won only one of five Premier League away games so far this season and missed out on the opportunity to top the table due to the surprise 2-0 loss, having previously gone nine games unbeaten and winning each of their last six.
And Mourinho was pulling no punches after the game, accusing his players of being too soft and lacking aggressive intent.
"Of course I am worried," he said at his press conference. "This season we have played five matches away – all difficult places to go – but we only have one victory and two defeats from those games so of course that concerns me. We were lacking everything – it was a really bad performance from us. We were not even thinking aggressively.
"We had the space to play the ball forward and between the lines behind Yohan Cabaye and Cheikh Tiote. We could have hurt the opponent and all we were doing is playing the ball from side to side – always horizontal, never vertical. We were slow and not sharp enough to attack spaces.
"In previous years the same performances have happened - Chelsea have been successful in cups but not in the Premier League. They have been finishing 15 to 20 points behind and one year they qualified for Champions League because they won it but finished outside the top four. Last year in December they were not in the title race."
Mourinho likened his side’s first-half performance to a friendly match, and says he will take some time to analyze what went wrong before looking ahead to future matches.
He continued: "The result is a consequence of one team not being there. When that is the case, they normally lose – so that is my explanation for the result.
"I think the first half was very easy to play – lots of space, no real pressure from the opposition. But we were not aggressive with the ball and often lost it straight away. We were not pressing. We were too soft, not aggressive nor intense enough. We gave them 45 minutes of a friendly.
"We deserved to lose. Maybe they did not deserve to win but we definitely deserved to lose."
The Portuguese boss said before the game he had been finding it tough to pick a team given the strength of the squad at his disposal and was asked afterward if he felt he made the right choices at St James' Park.
Mourinho joked: "I made 11 mistakes. I should have picked the other eleven rather than this one. That is the feeling I have when my team has played so badly.
"Of course I am exaggerating and some players had some normal or good performances but my feeling after a defeat is that I made 11 wrong choices. Before I think about how we will react I have to try to understand why we performed this way in this match."