Wagner has 58 test wickets at an average of 34.48 but is behind fellow pace bowlers Matt Henry and Doug Bracewell in the queue to join Trent Boult and Tim Southee in the starting XI.
Yet last year, Wagner produced stellar performances against India in Auckland and the West Indies in Barbados to bowl New Zealand to a couple of series-defining test victories.
He has largely bowled in the shadow of Boult and Southee, striving into the wind without a new ball. He fought the teeth of opposition middle orders but got few opportunities to devour their tails. As a result, he doesn't boast a sub-30 average.
Traditional statistics tend not to be a first change bowler's friend. They don't necessarily drill into the detail.
At Eden Park, with figures of four for 62 in the second innings, he bowled New Zealand to a 40-run win as India chased 407.
That day he had a fist pump and celebration to rival Mick Jagger as he cannoned through the Indian order like a rolling stone. Firstly, he eliminated Virat Kohli fortuitously at 222 for two, followed by century-maker Shikhar Dhawan.
Wagner was McCullum's stand-out player from that final day.
"He epitomised everything we want to be known for as a team and did it in clutch moments as well," McCullum said. "He hasn't always got the reward but has been an integral part of our jigsaw."
Wagner also benefited from the loyalty of coach Mike Hesson who observed him since his arrival in Otago during 2008.
He missed the first two tests away against the West Indies due to the use of two spinners, but returned in the deciding test with first-innings figures of 4-64 to restrict the hosts' lead to 24 runs. New Zealand won the test by 53 runs.
Wagner faces a big task to return, but refuses to give up.
"I'm hanging in there. Obviously it's disappointing and it hurts, but getting dropped is part of sport. I haven't played much in the last while, which makes it harder to do something."
Wagner joins the New Zealand A side for two four-day games against Sri Lanka A at Lincoln and Hagley Oval in late October.
"I need to look to what I can change and maybe push for selection in other international formats by playing more Twenty20, which I've missed by being with the test team. It gives me an opportunity to throw my name forward again."