Santner can trace his problems back to a bone defect he'd had for years.
"I must have done it when I was a teenager and thought nothing of it. It would give way sometimes, or get stuck when I tried to straighten it.
"I just thought that's normal, that's my knee.
"But over a period of time, it got more inflamed with the nature of how much cricket we were playing. It said, enough is enough.
"Then the bone started coming off and it turned bad pretty quickly."
Swelling meant he had to wear a knee brace. Getting the necessary amount of snap off his front or right leg at the bowling crease was uncomfortable.
"It was definitely affecting my bowling but I thought if I could get through the season and we'd see what's happened, but unfortunately it got to the stage where it needed surgery."
Santner drew an interesting analogy with a Crunchie bar.
"It's bubbly under the chocolate, so if the chocolate is the cartilage, there were a few bubbles under it, which have been scraped off and made nice and smooth. I've got a full range back into my knee and it should be better than it was."
Now he's walking, doing some cycling and gym work but it's very much at a measured pace. Santner is adamant he won't be rushing his return, and would rather make it one long layoff to fully fix the injury than risk a return to the operating table.
"The surgery went well and now it's trucking along really nicely. The swelling has gone and I'm getting a lot of movement back into the knee.
"It feels very good, which is the frustrating thing. I still can't do a lot and don't want to push it too quickly. We've got a plan in place."
The injury cost the left arm spinner and middle order batsman gigs with English county cricket side Derbyshire and Indian Premier League champions Chennai Super Kings.
In time, that might seem a small price to pay to get himself to a point where he can face the future with confidence.
His golf has been put on hold, and any golfer on scratch or a 1-handicap player would appreciate the frustration.
Santner has had plenty of time to ponder where his career is at during his layoff.
His bowling has long been tidy - averaging two wickets a test over his 17 matches, and one a game in ODIs - and at the time of the layoff, his batting was growing in productivity. Indeed his last four innings before the break, ODIs against England, produced scores of 45 not out (27 balls), 63 not out (62), 41 (54) and 67 (71).
His average is 25.47 in tests and 28.5 in ODIs. They need to rise but, at 26, he should be entering his prime and no longer simply be a young player of promise.
"I have to make sure I don't forget what I was doing," he quipped.
In his absence, Todd Astle played an important part in a test victory over England, and Ish Sodhi batted as if his life depended on it making an unbeaten half century to save the second test and secure the series in Christchurch.
Both are off to the United Arab Emirates to play Pakistan in two months, along with uncapped left arm spinner Ajaz Patel. Santner will sit and watch and wait.
"We're taking it scan by scan. There's no real return to play date yet, and we won't be rushed," Santner said. "I'd like to be playing as soon as possible but it's all about what the experts say."
