Zlatan Ibrahimovic's hat-trick earned United a handsome win in Thursday's last 32 first-leg match at Old Trafford.
Zlatan Ibrahimovic's hat-trick earned United a handsome win in Thursday's last 32 first-leg match at Old Trafford, but the scoreline did not tell the full story.
Saint-Etienne finished the first half strongly and substitute Nolan Roux spurned a glorious chance to level two minutes before Ibrahimovic netted United's second goal in the 75th minute.
Mourinho said United's first half had been "so bad" and when asked why, he said it had been down to a "lack of concentration".
"I had the feeling immediately in the dressing room: too noisy, too funny, too relaxed," he told reporters.
"Then my assistants had the feeling in the warm-up, with some of the guys not really focused on getting the right adrenaline in their bodies.
"The game started and the first thing we did was a back-pass (by Eric Bailly) to the striker to be face-to-face with Sergio (Romero, the United goalkeeper).
"Lack of concentration. When you don't have it, it's difficult to recover it. In the first half it was hard.
"Even myself on the touchline, it was difficult, the communication. I needed half-time. We were lucky to be winning 1-0. After that, different story, different result."
Mourinho has made no secret of the fact he would rather be playing in the Champions League than its sister competition, but he said that was no excuse for United's lack of focus.
"I know the Champions League is the competition everyone wants to play, everyone wants to win," he said.
"But we don't play the Champions League, we play the Europa League. We had the group phase a little bit like this. When we needed to win, we did it.
"When the focus is there, we answered. Tonight in the first half, it was too relaxed."
Ibrahimovic, 35, took his tally of goals against Saint-Etienne to 17 in 14 games, but the 17th hat-trick of his career was also probably the most scruffy.
He opened the scoring with a 15th-minute free-kick that was awarded for a seemingly non-existent foul on him by Jordan Veretout and his shot trickled past Stephane Ruffier via a deflection off Vincent Pajot.
His second goal was a tap-in created by substitute Marcus Rashford and his third, with two minutes to play, came from the spot after he crumpled following a challenge by Kevin Theophile-Catherine.
Ibrahimovic has scored more goals against Saint-Etienne than any other club and despite United's healthy advantage, he hopes to feature in Wednesday's return leg.
"Every time I've played against Saint-Etienne, with hard work there has been a couple of goals," said the former Paris Saint-Germain striker.
"I've scored a couple of goals tonight and hopefully I can do the same next week."
Saint-Etienne coach Christophe Galtier was unable to disguise his disappointment with Czech referee Pavel Kralovec.
"The scoreline is very harsh," he said.
"I'm disappointed by the appreciation of the game and the errors by the officials, in particular the first free-kick and the penalty.
"So I'm angry and disappointed for my players. They put so much into this game and they deserved to have more to play for in the second leg.
"It will be very, very complicated to turn this tie around. But we have to respect the competition and the people who will come to the stadium."
Saint-Etienne captain Loic Perrin was allowed to play on despite appearing to be left dazed following a clash of heads with his United counterpart Chris Smalling early in the second half.
But Galtier said he was satisfied with how his medical staff had handled the issue.
"There was a big clash of heads. But he didn't lose consciousness," Galtier said.
"If he had, the doctor would have taken precautions. The doctor said we could keep him on."