Rashford was spotted at Chinawhite club in Manchester on Sunday night hours after the 3-0 derby defeat.
A source close to Rashford, who turned 26 on Tuesday, said he attended “an intimate, pre-planned celebration in honour of his birthday at a private area of a Chinawhite organised by his friends.”
Ten Hag hinted the England international, who was named on the bench for the 3-0 Carabao Cup defeat to Newcastle on Wednesday, has been disciplined and said he would be available for the trip to Fulham on Saturday.
“I am aware of it [the party] and I spoke with him about it,” Ten Hag told a news conference on Friday.
“It’s unacceptable. I told him, he apologised and that is it. For us it’s an internal matter.”
Rashford scored 30 goals in all competitions last season and signed a new £315k-a-week contract over the summer. But he’s struggled for form so far this season, scoring just once in 14 appearances.
“He’s very motivated to put things right,” said Ten Hag.
“I know how much effort he is putting in. He’s totally with us. I see him every day in training, what he’s doing, I know. So he makes one mistake but also, off the pitch, what he is doing, how he lives, and I’m sure he is doing everything right to help the team and to perform and to let us win.”
United head to Fulham looking to bounce back from consecutive 3-0 home defeats to Manchester City and Newcastle. Ten Hag’s team are eighth in the Premier League table after losing five of their 10 games, but the Dutchman insists they will come through their poor run of form.
“I’m convinced, sometimes you have difficult periods but when you stick together — and we are together — then you come through those difficult periods,” he said.