On Friday evening, the Red Devils announced that they had reached an agreement with Los Blancos over the signing of the 30-year-old, who is expected to be unveiled to the English club's supporters before Monday's Premier League clash with Liverpool.
Casemiro gave an emotional press conference on Monday at his official farewell to Real Madrid and detailed his reasons for deciding to seek pastures new.
"As you know, I don't like to be the centre of attention. You'll surely understand that I'm speaking from the heart today. When I arrived here, I didn't know anyone in what was a new country. But, we built a life here," Casemiro said at his press conference.
"The titles I've won were achieved with the help of my family, the club, the president, Jose Angel Sanchez, Ramon Martinez, the coaches and my teammates. Of all my teammates, I can't not highlight Luka Modric and Toni Kroos. With them, I've enjoyed football so much.
"Thanks also to the fans, for those special nights. I'm sure that all I've learned here will help me to continue my career at another great club, at Manchester United. Thanks again. Hala Madrid!
"After the Champions League final I spoke with my agent and we had the sense that my time here was coming towards an end. My agent told me to go on holiday and relax and then we spoke again after a month and I still had this feeling that my cycle here was coming to an end.
"When the news came out then the transfer went quickly, but we'd already had these conversations. It was very difficult to speak about this with the president, but I think being honest is one of my great values. I was always honest in these conversations.
"Those who think that don't know me. I'm not like that. If it was for money, then I could have left four or five years ago. The club always acted well with me. It was my decision. It was simply the feeling that my cycle here had come to an end. This was clear to me."
"I feel very happy about all I achieved here. When I left Sao Paulo in January of 2013, I told myself I had to give my all for this club, in games and in training. I've tried to do that. I always slept well because I know I gave my all."
Casemiro has also revealed that he is looking forward to testing himself in the Premier League and is also hopeful of playing alongside Cristiano Ronaldo once again.
Ronaldo continues to be heavily linked with a move away from Old Trafford, with the 37-year-old determined to continue playing Champions League football this season, and it remains to be seen whether he stays with Erik ten Hag's side past the transfer deadline.
"The Premier League is a competition I've always liked and always wanted to play in. What I'm looking for is to have a new challenge, a new league and a new culture," the South American added.
"It's true that they're not in the Champions League right now, but I'm excited. I haven't spoken with Cristiano Ronaldo yet, but I'm excited to play with him again.
"I'm so keen to get started, with the biggest club in England. If it was up to me, I'd play for them already tonight against Liverpool. But, I know I haven't won anything there yet."
Casemiro is expected to train with his new Man United teammates this week with a view to making his debut against Southampton in the Premier League on Saturday.