When coach Eddie Jones named his first training squad McDermott wasn't in it. Now, after injury blows, he's the starting halfback and the captain.
He steps in with flanker Michael Hooper and props James Slipper and Allan Alaalatoa, having preceded him.
Jones explained his reasoning for McDermott's advance.
"Watching him closely at Super Rugby level, he captains a young Reds team, and now, coming into the team as the starting nine[halfback], he's got great potential, a calm head but shows good direction on the field and he's got all the attributes of a good captain.
"He's got a strong determination to win, he's a tough little bugger which are all good attributes of captains at nine."
That was proved after the superb tackle put on him in Melbourne last weekend by lock Scott Barrett. From a goal line lineout, McDermott received the ball and Barrett a fraction of a second later, resulting in a spilt ball and the game's opening try to All Blacks flanker Shannon Frizell.
McDermott said Jones told him that he wasn't playing well enough in Super Rugby when he missed the first squad.
"Of course, I was disappointed, but it gave me the time to work on that stuff, freshen up and get stuck into what I needed to work on.
"As the captain, I tend to look at the negative parts of my game, or the team, so taking a step back from that and looking at how we can grow and develop as an individual and a side, it gave me the time to nitpick through that."
Jones said McDermott had been willing to work to improve his game, and the starting role and captaincy were his reward for that.
McDermott said, "Being in this team, particularly from a leadership point of view, just how quickly we can solve problems is the biggest one I've learned.
"I'm the fourth captain in four games so that speaks of the quality I have around me in that space."
Jones said the captaincy for the World Cup was still being determined, but the leadership aspect of the side needed to change.
"This has been a period where we've been finding a lot out about the team, what's good, what's not so good, where we are and aren't strong. The nucleus of any good team is the leadership, and the captain's a massive part of that.
"It's all part of a transitional period for us."
All Blacks vs Australia: Saturday 5 August, 2.35pm, Forsyth Barr Stadium, Dunedin.