Dean Henderson has won one cap for England
The England international, 25, joined promoted Nottingham Forest on a season-long loan deal after failing to usurp David de Gea at Old Trafford.
Henderson said he was told he would "be the number one" last term, but missed Euro 2020 through injury and contracted Covid-19 at the start of the campaign.
The goalkeeper said he was left "fuming" over his lack of game time.
Henderson, who added that he has not spoken to Erik ten Hag since the manager's arrival in May, told Talksport: "To be honest, it has probably been the toughest 12 months of my career.
"It was frustrating, because I turned down so many good loans last summer for that reason, and they would not let me go."
Henderson became first choice under Ole Gunnar Solskjaer over the second half of the 2020-21 season, but was not picked for the Europa League final against Villarreal.
The start of the following season was disrupted and he didn't get a look-in under Solskjaer or interim boss Ralf Rangnick, featuring in just three cup games in the entire campaign.
Henderson added: "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.
"To sit there for 12 months, it is criminal really, at my age. I was fuming. 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.
"I worked hard on and off the pitch to keep improving day in, day out, and now I am looking forward to the season with Nottingham Forest."
Forest begin the new campaign against Newcastle on Saturday (kick-off 15:00 BST).
-- Courtesy of BBC Sport