The France international accused the Premier League outfit of being "disrespectful" towards him, and making him train alone after he spurned a new deal.
"They left me out because they said I wanted to leave. That is disrespect," Pogba told BBC Sport. "Maybe they didn't think if they were making a mistake.
"I was training alone for one week. They told me to go in the gym. I said 'I am not an athlete, I am a footballer.'"
Pogba, who made just seven substitute appearances during his time in England, left United in July 2012.