Sancho has not featured since October and missed United's training camp in Spain during the World Cup, instead travelling to the Netherlands to take part in an individual training programme.
The 22-year-old has now come back to Manchester but Ten Hag says the England winger is not fit enough to return to action, while also suggesting the problem is more than just a physical one.
"In this moment he is not fit enough," Ten Hag told a news conference on Thursday.
"It's physical, but physical is also the connection with mental. But I think he is now making good progress on the physical part and that will help him. I hope he can return quickly but I can't say a duration of how long it will be."
Ten Hag is hoping to sign a forward in January to compensate for the departure of Cristiano Ronaldo.
Sancho's absence has further reduced his options up front but Ten Hag insists he will not rush the former Manchester City and Borussia Dortmund man despite facing a busy run of fixtures in four competitions in the second half of the season.
"I would like to have Jadon back as soon as possible," said Ten Hag.
"But also in this, I can't force this process and so I will do everything that's in my power but some processes you can't force and this is one of them.
"So I have also to show patience, although I don't have the patience because yeah, you are right, we have a lack of opportunities in the frontline, players who are available and capable to contribute in the Premier League.
"Jadon is one, when he is fit, he will contribute and then we have an extra option and so we will have more chance of winning a lot of games.
"We have some hurdles to take but I think he's in a good direction. And what I say I can't force this process and so I don't do [it]. But I would be really happy when he returns to the squad for team training, so that will be the next step."
United have been keen to stress that Sancho has not been axed from the squad and that the decision for him to train away from the main group was mutually agreed with Ten Hag following a series of meetings held before the World Cup.
Sancho has been involved in small group sessions at Carrington this week and Ten Hag believes he has benefitted from his time away from the club.
"He was in the Netherlands but also the process could have been in England as well," said Ten Hag.
"But the good thing is he is back in Carrington and that shows he is making progress and he is ready for the next step.
"I think many top athletes, in football and also in other sports, sometimes it's good to go away from the place where you are at daily to get a new vibe and a new experience. People have a different approach and this can give you the right push to get back on track.
"Football players aren't robots. No one is the same. I think for everyone, you need an individual approach. We thought that, in cooperation with Jadon, it was the best choice."