Sancho has undergone his medicals with United on Tuesday, ahead of the completion of his £73.1m switch from Borussia Dortmund winger this week.
The 21-year-old’s versatility will see him compete for a place in Ole Gunnar Solskjaer’s wide areas with Anthony Martial, Dan James, Rashford and Greenwood.
Mourinho believes Greenwood, who missed out on a place in England’s Euro 2020 squad, is capable of making next year’s World Cup squad but is concerned the 19-year-old could be ‘hidden’ due to the competition for places at United.
“The situation at Manchester United will be interesting from an England point of view.
“Jadon Sancho has gone to United, Marcus Rashford is also there and Mason Greenwood is a good example of a player who wasn’t in this squad but can be in Qatar when he will be much more mature and has a good possibility of being selected.
“Three young England players in one club, three with potential to play in the national team.
“Which one will have an amazing season but which one could be hidden, not starting many matches because of the coach’s decision? Greenwood has the talent to be there,” Mourinho told The Sun.