Krul joined Newcastle from Den Haag in July 2005, aged 17.
Harper said: "Shay Given and I were the two experienced keepers at the time but from day one, you could tell this kid had enormous self belief and drive.
"He was very confident but at the same time, he was willing to listen and take advice.
"It was only a matter of time before he made it on the big stage - and unfortunately, he ended up taking my place.
"I'm absolutely delighted for Tim. He's a smashing lad and having watched him progress through the years, I had a real sense of pride seeing him excel on the biggest stage of all."
For the big stories and the banter follow us on Twitter: @tribalfootball