The £73m signing was omitted from the squad for the 3-1 defeat by Arsenal when the Dutchman said he had not performed well enough in training.
The winger, 23, wrote: “Please don’t believe everything you read! I will not allow people saying things that [are] completely untrue. I have conducted myself in training very well this week. I believe there are other reasons for this matter that I won’t go into, I have been a scapegoat for a long time which isn’t fair!”
While Sancho has since deleted the tweet, he is unavailable for selection as United determine whether and how to punish him.
A club statement read: “Jadon Sancho will remain on a personal training programme away from the first team group pending resolution of a squad discipline issue.”
United rejected an offer from Saudi Arabian club Al-Ettifaq to loan Sancho after the European transfer window had closed, but were only prepared to let him leave if he was sold for a suitable fee.
Sancho sat out three months of last season for United when Ten Hag omitted him for what he called “physical and mental issues”, which included sending the Englishman to the Netherlands for individual training.
United are also without another winger, Antony, who was given a leave of absence while he addresses allegations of assault from three women. The Brazilian has maintained his innocence, and police in Sao Paulo and Manchester are investigating, though no charges have been brought so far.