Sancho was dropped for United's 3-1 defeat to Arsenal earlier this month for what manager Erik ten Hag (53) deemed poor performances in training.
He responded by posting on social media that he had been made "a scapegoat for a long time."
In a statement, United said: "Jadon Sancho will remain on a personal training programme away from the first-team group, pending resolution of a squad discipline issue."
Sancho, who cost United £73million when he joined from Borussia Dortmund two years ago, has struggled to live up to that price tag at Old Trafford.
He has scored 12 goals in 82 appearances, but was not always a regular starter during Ten Hag's first season in charge.
Sancho's absence compounds a miserable start to the season for United on and off the field.
Ten Hag's men have lost two of their opening four Premier League games.
The Red Devils are also without Brazilian winger Antony after he was given a leave of absence to fight allegations of domestic abuse made by his former girlfriend.
