Phelan worked with the Leicester City defender when in charge of the Tigers.
"His body was maturing. Everything is a bigger deal now - diet, sleep. Information is at hand to make you a better specimen. It is not just physical but mental. Harry had to learn bits of that, and he did.
"He is going to learn a lot from the World Cup that's for sure, because there will be teams England come up against, hopefully, who know he will bring the ball out all the way down that left hand side."