The veteran midfielder also offered his clearest endorsement yet of Michael Carrick as the club's permanent manager.
The 34-year-old Brazil international, who joined United from Real Madrid for around €70 million in 2022, has made 159 appearances across four seasons, scoring 26 goals and winning the League Cup and FA Cup. He announced in January that this season would be his last, citing a desire to leave on his own terms rather than wait to be moved on.
Fan support for an extension has grown markedly since Carrick's appointment in mid-January. Manchester United supporters have chanted for one more year at recent matches, and Casemiro's wife was visibly moved by the crowd's affection following the 3-2 victory over Liverpool. Teammate Matheus Cunha also suggested the situation was not entirely resolved.
Casemiro brushed all of it aside.
"There's no possibility of staying one more year. They were four beautiful, wonderful years, and I'm eternally grateful not only to Manchester United but to the fans. My cycle has ended here. I want to enjoy every moment and I will be a United fan for the rest of my life."
He also addressed the remarkable trajectory of his final season, in which he went from fifth-choice midfielder under Rúben Amorim in the autumn to a first-team regular and key figure under Carrick.
"It may have been one of my greatest achievements. The most normal thing would have been to say the coach isn't using me, I've won enough, I'll see out the contract. Instead, I kept working and came back. I'm proud of that."
His endorsement of Carrick was unambiguous.
"He completely deserves the job permanently. What he has done here since January has been extraordinary. United needs a manager who understands the club, and Carrick does."
Casemiro is expected to become a free agent in July, with clubs in Major League Soccer and in European football both monitoring his availability.