Ten Hag left Sancho out of his squad for the loss at Arsenal earlier this month, saying the winger's training performances had been below standard.
Sancho, 23, rejected the Dutchman's reasoning and claimed he had been "a scapegoat for a long time".
United said in a statement Sancho would be on a "personal training programme away from the first team".
The statement added that the decision was made "pending resolution of a squad discipline issue."
The news comes in a week where another United forward, Antony, also stepped away from first-team duties amid domestic abuse allegations made by his former partner.
Sancho moved to Old Trafford from Borussia Dortmund for £73m in 2021.
But he has struggled for consistent form since making the switch, and has scored only nine goals and provided just six assists in his 58 Premier League appearances.
Last season Sancho spent three months battling physical and mental issues, with Ten Hag at one point saying the England international was not fit enough to play.
He returned to action in the EFL Cup semi-final win against Nottingham Forest on 1 February and a week later sparked a comeback against Leeds United in the Premier League, scoring the equaliser in a 2-2 draw.
Ten Hag spoke of his pride in "magnificent" Sancho in the aftermath, but Sancho's involvement has lessened since then.
He has made three substitute appearances this season with United winning two and losing two of their opening four Premier League fixtures.
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