United all but sealed their place in the round of 16 in the Europa League thanks to a Zlatan Ibrahimovic hat-trick.
But the French side troubled United in the opening 45 minutes with several efforts on goal.
And Mourinho believes the atmosphere in the United dressing room before the match contributed to their sloppy display.
Manchester United player ratings against Saint-Etienne in Europa League
Thu, February 16, 2017
Express Sport brings you Manchester United's player ratings against Saint-Etienne in the Europa League
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Express Sport brings you Manchester United player ratings against Saint Etienne
Manchester United manager Jose Mourinho
He said: "Lack of concentration. I had the feeling immediately in the dressing room, too noisy, too funny, too relaxed. My assistants had the feeling in the warm-up, some of the guys were not focused on getting the right adrenaline in their bodies.
"The game started and the first thing we did was a back-pass to the striker.
“The first-half was hard. Even myself on the touchline, it was difficult to communicate. Second-half different story.”
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