Henderson, who has been with Erik Ten Hag's side since the age of 14, recently embarked on his latest loan spell away from Manchester United, signing for newly-promoted Nottingham Forest for the 2022/2023 season.
Henderson has been unable to displace David De Gea as Manchester United's first choice since returning from a two-year loan at Sheffield United in the summer of 2020.
The 25-year-old made 26 appearances in all competitions for the Red Devils in the 2020/2021 season but featured just just three times across all competitions last term, with none of them coming in the Premier League.
Henderson claims he was promised the number one jersey at Manchester United last season but that never became a reality after missing the European Championship through injury and getting a bout of Coronavirus.
"To be honest with you, it's probably been the toughest 12 months of my career," Henderson told TalkSport.
"I'm so happy I've got out the other end of it now, and I'm excited to be here and ready to go.
"The conversation I had [with United following Euro 2020] was 'you're coming here to be the number one goalkeeper'. I got COVID, come back, so I should have still been the number one, but then unfortunately no one followed through with anything.
"It was so frustrating as well because I turned so many good loans down last summer for that reason, and they wouldn't let me go, so it was frustrating. To sit there and waste 12 months is criminal really at my age, I was fuming.
"But I worked hard off the pitch and on the training pitch to keep improving day in, day out, so I'm really excited to be back here and looking forward to the season with Forest."
Henderson has a contract with Manchester United until the summer of 2025.
