Erik ten Hag has branded as “unacceptable” Marcus Rashford having a 𝐛𝐢𝐫𝐭𝐡day party at a nightclub a few hours after Manchester United’s 3-0 defeat by Manchester City.

Ten Hag’s side lost the derby on Sunday afternoon and that night Rashford attended an event at Manchester’s Chinawhite to celebrate his 26th 𝐛𝐢𝐫𝐭𝐡day, which was on Tuesday.

“I am aware of it, I spoke with him about it,” Ten Hag said. “It is unacceptable. I told him, he apologised and that is it. It is an internal matter.” The manager was asked whether Rashford had been fined. “It is an internal matter,” he said.

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Last season Ten Hag dropped Rashford from the starting XI at Wolves owing to late timekeeping. In light of this and how United are struggling, Ten Hag was pressed on whether Rashford’s conduct on Sunday showed the forward was out of tune with the manager’s demands.

“Your suggestion is not right,” Ten Hag said. “He is very motivated to put things right. I know how much effort he is putting in. He makes a mistake but that doesn’t say [mean] he is not fitting in. He makes one mistake but also off the pitch what he is doing, how he lives, I am sure he is doing everything right to help the team.”

Ten Hag said Rashford is available for Saturday’s trip to Fulham but United will be without Casemiro for several weeks after the midfielder sustained a hamstring injury. Last Sunday’s defeat was followed by another 3-0 loss, to Newcastle in Wednesday’s Carabao Cup tie, which knocked the holders out. Ten Hag denied Rashford had started that game as a substitute to discipline him, pointing to how Bruno Fernandes and Rasmus Højlund were also on the bench. “No, you have seen we rotated more,” he said.

The defeats by City and Newcastle compound a poor opening phase to the season. United have lost five of 10 home games, won one of three Champions League matches and are eighth in the Premier League, eight points off fourth spot and seven behind fifth-placed Aston Villa. Fifth could secure Champions League qualification.

Since Sir Alex Ferguson retired in May 2013, David Moyes, Louis van Gaal, José Mourinho and Ole Gunnar Solskjær have failed to make United title contenders. Ten Hag was asked whether the structure of the club made this impossible.

“I wouldn’t work here if I didn’t think it wasn’t right,” he said. “Sometimes you have difficult periods but when you stick together you come through those difficult periods.” Ten Hag denied United had to beat Fulham due to his job being under threat. “I never think in such scenarios – we have to win every game,” he said, insisting he trusted his players to turn the campaign around.

“I trust the players and I have to take the responsibility [too],” he said. “Remember the fight we showed against Brentford [scoring twice in added time to win] – the comeback. If there was no fight or spirit or characters in the dressing room you couldn’t do this. They are positive. They want to put this right. We know the standards here and we have to match the standards every day. We had two big setbacks but we will fight back. This dressing room is strong, the staff is strong and the manager is strong to put it right.”

Sir Jim Ratcliffe, who is expected to buy a 25% share in United for £1.3bn, is understood to be proposing to invest an initial £245m towards making Old Trafford a world-class stadium again.