The England U21 international is set to sign with the English champions in a £16million deal and Redknapp has rued the fact that he was unable to lure him to White Hart Lane.
"I think he's an amazing young talent and he could easily go on and become an England centre-half in years to come," Redknapp told talkSPORT.
"I think he's a great singing for Man United.
"We're struggling to find people that are better than what we've got. It's not easy unless you pay massive money and massive wages."