Sancho has been flirting with a move away from Signal Iduna this summer as big guns in English football are said to be hoping to secure his services.
The 20-year-old has been regarded as one of the best forwards in Europe since donning the famous black and yellow shirt in 2017.
Sancho has netted 17 goals and provided 19 assists in 31 games in all competitions for Dortmund this season.
Speaking to The Football Show on Sky Sports, Redknapp believes his compatriot would soon become one of the top five players in the world, and has backed the Red Devils to sign him.
"His next move is going to be so important because he has to make sure he gets in that team and plays regularly because he is an amazing talent, one of the best I've seen in a long time,” the former Liverpool man said.
"We're talking about one of the top five players in the world in a few years, he has that much ability. If he goes to United, everyone knows the pressure of playing for a big club, but he looks he can handle the pressure and it would be a brilliant move if he goes there”
He added: "Would a move to Manchester United be a good move for Sancho?"
"We're talking about an incredible talent, someone who has made some really good decision in moving abroad.”
"He took a real risk and it's paid off; he's playing in the Champions League and has played amazing football for club and country.”
