The England player was forced to train away from Manchester United's first team because of a "discipline issue" following his claim on social media he had been made a "scapegoat" after being cut from United's squad for the 3-1 loss to Arsenal this month.
He is now on a "personal training program" until the matter is resolved, United said.
Ahead of Saturday's Premier League home game against Brighton, ten Hag was asked directly whether Sancho would put on the United jersey again.
He answered: "I don't know. I am sitting here. Tomorrow we have a big game, we are going into a new block of games, many games in a condensed program. I focus on that."
He added: "He is not available, so in this moment, he is not important, because he can't contribute."
Ten Hag, who has previously taken a hardline stance after transgressions by Cristiano Ronaldo and Marcus Rashford, declined to go into detail about Sancho. But he made it clear he intended to stamp his authority on the club.
"Strict lines is what the club asked me because there was no good culture before I entered last season," the Dutch coach said. "So to set good standards, that is what I did and it is my job to control the standards.
"Of course, it is never (the case that) someone only makes one mistake, it is a whole process before you come to a certain outcome about strict lines. If (it is) staff or players or whoever, there is a structure to cross lines so you have to be strong."
Ten Hag had said Sancho was dropped from the Arsenal game because of his performances in training.
Sancho gave his opinion in a post on X, formerly known as Twitter, when he said he would not allow people to say things that were "completely untrue."
"I believe there are other reasons for this matter that I won't go into," he said. "l've been a scapegoat for a long time which isn't fair!"
The situation with Sancho is not the only off-field issue that has cast a shadow over United's season.
Mason Greenwood was allowed to join Getafe on loan after he and United mutually agreed to part ways six months after a criminal investigation of attempted rape was closed by prosecutors.
Brazil winger Antony is currently on leave while he addresses allegations of domestic abuse.
Ten Hag said he had "no idea" how long Antony would be unavailable.
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