The former Real Madrid star will don the No18 short for United, the number formerly worn by Paul Scholes.
During his fabled career at the club, Scholes sported an array of numbers on his back with No18 the most prominent, between 1996 and 2011. Bruno Fernandes is another player to have worn the number in the past, alongside Ashley Young, before switching to No8 in the summer.
Casemiro has been officially unveiled as a United player following an agreement with Madrid that will see him earn a hefty £350,000-a-week wages at Old Trafford.
But the player has insisted he was always looking for a new challenge this summer.
“I think there are a few who don’t know me,” he stated. “It’s not about money. They’re wrong. That’s not the case.”
He added: “After the Champions League final I had the feeling that my cycle was over. I’m honest. After the holidays, with a clear head, my feeling was the same.
“It wasn’t that fast. I had already spoken with the club as soon as we finished the Champions League. My wife supported me from the beginning. My family was always by my side in this decision.”