The Tykes thought they had done enough to retain the services of the promising full-back as the January transfer window edged towards a close.
Their hand was forced late on, though, with Everton and Wigan battling it out for the 18-year-old's services.
Flitcroft feels the Toffees have landed the most promising defender in the country and concedes there is no chance of him finding a replacement of similar quality.
He said: "On Wednesday, I was pretty confident we would keep John.
"But then I got a call on Thursday morning to say he would probably be leaving.
"To be honest, I am numb. It is like losing a member of your own family.
"I was asked if I could replace him, and the answer was, 'no'. How do you replace the best young defender in England?
"There is nobody out there as good as John, and that is why clubs were prepared to pay £3million for him."