The Premier League giants confirmed the signature of Sancho from Dortmund on Friday on a five-year contract with an option of one further year.
At £73m he is the fourth most expensive signing for Manchester United behind Paul Pogba, Harry Maguire and Romelu Lukaku.
Manchester United wanted to sign Sancho last summer and had him pencilled in for the iconic number 7 jersey worn by greats such as George Best, Eric Cantona, David Beckham and Cristiano Ronaldo.
But Cavani took up that squad number and it has been claimed that Sancho will wear the number 25 jersey.
The Uruguayan won Manchester United hearts last season and will continue to wear that iconic number.
The 21-year-old winger is likely to inherit the number 7 once Cavani leaves Manchester United.
Another former Manchester United winger in Antonio Valencia wore the number 25 before becoming the number 7 later in his Old Trafford career.
