The Portuguese berated his players over their poor preparation for the game at Old Trafford.
"I had the feeling immediately in the dressing room: too noisy, too funny, too relaxed," he said. "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 to their striker to be face to face with our goalkeeper. It was a lack of concentration. When you don't have it, it's difficult to recover it. In the first half it was hard."
United went in at the pause thanks to Ibrahimovic's goal from a free kick in the 15th minute.
His second in the 75th minute was a tap-in after being set up by substitute Marcus Rashford and his third, with two minutes to play, came from the penalty spot after he was felled by Kévin Théophile-Catherine.
Most goals at Saint-Etienne
Ibrahimovic has scored more goals against Saint-Etienne than any other club.
"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 and hopefully I can do the same next week."
Saint-Etienne coach Christophe Galtier hit out at the officials following the match.
"I'm disappointed by the appreciation of the game and the errors by the officials, in particular the first free-kick and the penalty,"he complained. "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."