"I was at Arbroath last week for a reserve game and it was the second coldest day of my life," said Calderwood.
"The wind was howling, the keeper couldn't clear the ball out of his own box yet Josh was everywhere, trying to get on the ball or taking free kicks.
"He's just 18 yet he showed exactly why he captained England at every youth level.
"He has quality, physical presence and just loves playing this game. He has everything about him that is needed to be a top player.
"But most of all he's a winner with the hunger to succeed."
Walker inked a new, long-term deal with Boro days before he joined the Dons on-loan.