Dean Henderson described Manchester United's treatment of him last season as "criminal" and said he was "fuming" at his lack of playing time.
The 25-year-old England goalkeeper, who joined promoted Nottingham Forest on loan last month, was unable to displace David de Gea as first-choice goalkeeper, making just three appearances for United last season.
Henderson said he was told he would "be the number one" last term after making 26 United appearances in 2020/21 having impressed the previous season on loan at Sheffield United.
But he missed Euro 2020 through injury and contracted Covid-19 at the start of the campaign.
"To be honest, it has probably been the toughest 12 months of my career," Henderson told talkSPORT radio on Tuesday. "The conversation I had coming out of the Euros squad was: 'You're coming back here to be the number one.'
"I got Covid-19, came back, so I should have still been the number one but then nobody followed through with what they had told me. It was frustrating, because I turned down so many good loans last summer for that reason, and they would not let me go."
Henderson said he has not spoken to Erik ten Hag since the Dutchman was named United manager in May and that his treatment at Old Trafford had left a bitter taste.
"To sit there for 12 months, it is criminal really, at my age. I was fuming," he added. "I told the hierarchy that I need to be playing football and to let me go, and I was almost gone before the manager [Erik ten Hag] came through the door. I have not spoken to him since."
Forest begin their Premier League campaign with a trip to Newcastle United on Saturday. United's season kicks off at home to Brighton & Hove Albion on Sunday.