"The more overs I bowl, and the more my spine is moving, agitates my spinal cord and it reaches a point where it malfunctions and I get these pains in my legs."
Mills admitted it was hard news to take at 22.
"It was pretty nasty. I had all the tests done and I was offered retirement at 22, which you can imagine is not what I wanted to hear when I was in and around the England stuff," he said.
"It was a bad time, tough to take but I started to go down a path of giving T20s a crack. That was two years ago, it's gone well and now I'm on a journey where hopefully I will play for England in T20s and travel the world playing T20 competitions."
The next few months pan out like this for Mills: he's just come from the Bangladesh Premier League, where he was a teammate of New Zealand's Grant Elliott, and after his stint with Auckland he's off to join England for their three T20s against India; then it's Dubai for the Pakistan Super League, and hopefully a gig at the Indian Premier League, and on to the English T20 domestic competition.
He knows he needs to be careful.
"You can't just flog yourself. Your bowling loads are down but the amount of travelling is going to be even higher and that puts a different strain on your body. In the long run, hopefully it will give me the best chance of playing cricket for as long as I can."
Yorkshire-born Mills has played one T20 for England, against Sri Lanka last July. In 43 T20 matches, he's taken 52 wickets at 20.88, with an impressive economy rate of 7.67.
He's caught the eye during the McDonalds-sponsored Smash with his clever changes of pace, ranging from deceptively slow to distinctly sharp, and a runup which starts with a quirky hop.
"I had a knee injury about 18 months ago, and shouldn't probably have been playing. I could bowl, but couldn't sprint very well. I couldn't accelerate at the start so I had to do that to get a rolling start to get myself going," he laughed. "Now I can't get rid of it."
But it works, judging from his form.
He has taken six wickets at 20.83 apiece and with a 6.63 economy rate in five games for Auckland going into their match against Wellington today at Eden Park.
Auckland captain Rob Nicol is delighted to have him in the side.
"There's a point of difference with his skills and we're very lucky to have him."