Sancho was not named in the United squad for the trip to the Emirates Stadium which ended in a 3-1 defeat.
Quizzed over the 23-year-old's no-show against one of their bitterest rivals, Ten Hag revealed Sancho's application in training last week was not up to the required standard which led to him getting dropped.
Explaining his decision not to pick Sancho for his squad to travel to Arsenal, Ten Hag said: "Jadon, on his performances in training we did not select him.
"You have to reach a level every day at Manchester United and we can make choices in the front line. So for this game he was not selected."
But Sancho has taken to social media to issue a quick rebuttal to the claims made by his manager.
Writing on X, formerly known as Twitter, Sancho said: "Please don't believe everything you read! I will not allow people saying things that is (sic) 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, l've been a scapegoat for a long time which isn't fair! "All I want to do is play football with a smile on my face and contribute to my team.
"I respect all decisions that are made by the coaching staff, I play with fantastic players and (am) grateful to do so which I know every week is a challenge.
"I will continue to fight for this badge no matter what!"
Sancho has made three substitute appearances in United's opening four league games so far this season.
He joined from Borussia Dortmund for £73m in the summer of 2021 but has scored just 12 goals in 82 appearances.
