Rashford's disputed red card in Copenhagen on Wednesday contributed to a dramatic United collapse as they blew a 2-0 lead and conceded two late goals to lose 4-3, dropping to the bottom of Group A in the Champions League with two games left.
Gareth Southgate kept faith with the 26-year-old by naming him in his England squad on Thursday, but a player who scored 30 goals for United in all competitions last season has only one from 15 appearances for the club this term.
"I think he's not happy, we are not happy," Ten Hag said of Rashford's performances. "We have an expectation. He has high expectations from himself. In this moment he is not in the best form but I know he will be back.
"I know when the team is playing better he will play better. He will go and score goals. I am confident of that. This season he will improve and score goals. He is totally in the team, he is aware of everything so I think he will be back on track.
"It can happen very quickly and sometimes you only need one game. I'm sure he will get there."
Mason Mount is been another player in the spotlight, with the 24-year-old midfielder struggling to make an impression since his £55 million ($67 million) switch from Chelsea in July.
Mount made his first start since early October in the 3-0 League Cup defeat to Newcastle on November 1 but was back on the bench against Fulham and Copenhagen, with questions being asked about his role in the side.
However, Ten Hag said there had been no change in his expectations of the player.
"It hasn't changed at all," he said. "He started the season and then he got injured. It's the worst moment for a new player to get injured because it stops the integration process. That was definitely a setback for us all and for him now he has to fight to get his place back."
United's midweek defeat came at an extra cost as Jonny Evans limped off early after pulling up off the ball.
The 35-year-old, who has been a regular starter in recent weeks, has been ruled out of the Luton match, adding to Ten Hag's defensive woes.
