José Mourinho rebuked his Manchester United players for lacking concentration and being too “relaxed and funny” in the dressing room before St-Étienne were beaten 3-0 in the Europa League last-32 first leg .

The manager was delighted with the result, which was due to a Zlatan Ibrahimovic hat-trick that took him to 23 goals, the most in a season for United since Robin van Persie scored 30 in the club’s last title-winning campaign, 2012-13. But Mourinho was particularly unhappy with his side’s attitude beforehand and felt he had to change it during the half-time interval.

Anthony Martial catches the eye for United as he feels his way back to form | Paul Wilson Read more

He said: “I am delighted, especially because the first half we played so bad and managed to finish the first half winning 1-0 when we didn’t deserve to, so I was pretty confident the second half we had to improve. It was down to lack of concentration, I had a feeling immediately in the dressing room – too noisy, too funny, too relaxed.

“My assistants had the feeling in the warm-up with some of the guys not really focused, not 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 and one-to-one with Sergio [Romero]. When you don’t have concentration it’s hard to recover it. Even on the touchline I felt the communication was difficult so we needed half-time. The second half was a different story, a different result.”

Mourinho did not accept that playing in Europe’s second-tier club competition was an explanation. “I know the Champions League is a competition that everyone wants to play and win,” he added. “But we don’t play Champions League, we play Europa League. We had a group phase a little like this – when we needed to win we did it.

“When the focus is there we answer. Tonight, the first half was too relaxed. St-Étienne had the right intensity and created problems. If the first half had ended 1-1 the game may have been different. If we progress and we beat St-Étienne and go into the last 16 then people will have the right attitude. So let’s do next Wednesday [in the return leg], [get] the right result and go for the next draw.

“To score three goals, not concede at home, it’s a good result. It’s not over but it’s a good result. [Ander] Herrera is suspended [after being booked]. I always respect football, it can be nasty with you. I don’t want my team to be punished so I will go with a strong team.”

Ibrahimovic’s hat-trick was the 17th in his career and also took him to 17 career goals against the French club. “Every time I’ve played against St-Étienne, with hard work there has been a couple of goals,” he said. “We’re winning but in a short time everything can change. It’s important to keep getting the wins we need. Everything can change but we’re happy at the moment. This is the decisive moment for the season. We are still in all four competitions. The fifth we already won [the Community Shield].”