The Dutchman, though, insists it is only a matter of time before Rashford returns to his best and starts firing on all cylinders.
Rashford netted 30 goals in all competitions last campaign but has managed to find the back of the net just once so far this season.
Man United have made their worst start since 1974, with nine losses in 17 games to date.
"I think he's not happy, we are not happy. We have expectations, he has high expectations of himself," Ten Hag told reporters ahead of his side's clash with Luton on Saturday.
"At this moment, he is not in his best form. But I know he will be back. I know when the team is playing better, he will play better, he will score goals. I am confident of that.
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"This season he will improve and score goals, he's totally in the team and aware on everything. He will be back on track.
"That can happen very quickly. Sometimes you need only one game, I'm sure he will get there."
United have lost three of their last four matches, including their last two on home soil, but Ten Hag insists his side can turn things around.
"We are very disappointed to lose any game but finally it's about the end result," Ten Hag added.
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"It's always about a process, thinking about a process and managing the process. That's the only thing I focus on.
"We often proved that we can, like [in the recent wins against] Fulham at Brentford, overcome big setbacks."
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Ten Hag also touched on the absence of Mason Mount, who has suffered an underwhelming start to life at Man United.
The former Chelsea star joined the club in a £60m deal in July, and has started just one of United's last six clashes.
Mount has also failed to register a goal or assist in his first seven appearances for the club.
"No. Hasn't changed at all," Ten Hag said on Mount's form.
"But he started the season, then he got injured, it's the worst moment for a new player to get injured because it stops the reintegration process and that was definitely a setback for someone like him. And now he has to find his place back.
"He had some starts, also recently, last week against Newcastle, for instance. But there are always reasons and sometimes the injury, then other players coming in doing well, you have to wait for your time to play."