A hat-trick from striker Zlatan Ibrahimovic gave the scoreline a flattering look after the Premier League side survived several nervy moments during the match at Old Trafford, particularly in the first half.
"I am... especially because in the first half, we played so bad, and we managed to finish it winning 1-0 when we don't deserve. It was down to lack of concentration," Mourinho told reporters when asked if he was happy with the result.
"I had the feeling immediately in the dressing room... too noisy, too funny, too relaxed. Then my assistants had the feeling in the warm-up, with some of the guys not really focused on the getting the right adrenaline in their bodies.
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"Even myself on the touch-line, I felt it was difficult, the communication. I needed the half-time. I am not happy with it. At half-time, the result could be 1-1. I always think we have to play every game with the same attitude."
The Portuguese manager, however, did stress the second half was "a different story".
United, who have lost once in their last 23 matches, travel to face second-tier side Blackburn Rovers in the fifth round on the FA Cup on Sunday.
(Reporting by Shravanth Vijayakumar in Bengaluru)
