Zlatan Ibrahimovic's hat-trick proved the difference between the two sides and United will take a big lead into the second leg of their last-32 clash in France.
Mourinho felt his players didn't have the right attitude, however, and he says they were lucky to hold a 1-0 lead at the break against Les Verts.
"I am," he said afterwards when asked if he was happy with the result. "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.
"I had the feeling immediately in the dressing room [before kick off] - 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.
"Then the game started and the first thing we did was a back-pass [by Eric Bailly] to the striker [Romain Hamouma] to be face-to-face with [United keeper] Sergio [Romero].
"So, lack of concentration. And when you don't have it, it's difficult to recover it. So in the first half was hard.
"Even myself on the touchline, I felt it was difficult, the communication. I needed the half-time. So we were lucky, yes, to be winning 1-0.
"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."
Mourinho believes the tie is "not over" just yet ahead of the second leg in France and he is convinced his squad will step up their levels if they manage to progress in the competition.
He added: "I think if we progress and if we beat St Eteinne and go into last 16, then the people will have the right attitude. Lots of good teams are going to be in the next round.
"Big teams are there, Champions League teams' level will be there. So let's do next time the right result, let's go for the next draw and I think the motivation will come."