"The plan for Rehan was that we always looked to bring him into the squad," Key said in Abu Dhabi on Thursday. "This is the best way to aid his development. He is a serious talent, but he might be four or five years off being the finished product. He is nowhere near the finished article at the moment.
"But we just hope that being involved with the Test squad, with McCullum and Stokes and the mentality they have ... bowling at Joe Root. He becomes a full member of that squad, he's not just a net bowler. If needed, we believe he can play and do a good job for us with bat and ball. It's a chance to put him on a path that will get the best out of him.
"Credit to the Test set-up, we feel that is the best group of people to aid his development and get him to where we think he can be quicker, by being involved with not just the captain or Brendon, but every one of those players has a part to play in his development."
Ahmed's innings - bold, care-free, very much what you'd expect from an 18-year-old teeming with confidence - can perhaps be extrapolated to the impression he has made on McCullum, who was understood to be reticent about selecting Ahmed before he had seen or fully interacted with him. He has evidently been impressed over the last 48 hours.
Nevertheless, the duty of care towards a kid with just three first-class appearances for Leicestershire, who only turned 18 in August, was always part of the consideration.
"How we've done it, we wanted it to be more of a soft launch, rather than just announcing him in a squad and away you go, with all the media speculation," Key revealed. "He has been able to come out here, we have had a look at him. Mo Bobat [ECB performance director and head coach of the Lions] knows him very well and has had a big part in his development since being a young kid. Every one of these young players has come through that pathway with Bobat, David Court [Player ID Lead]. They have a good read on these people, they have been in touch with the families and all that stuff. That was the best way we felt we could have that soft launch, so he was around a bit before he finds out he's been picked in the squad."
Key cedes Ahmed "is going to have decisions going forward in his career and life", around which colour ball takes his fancy at various points of the year. And he has no qualms in admitting this exposure to Test cricket can sow a sizeable seed in Ahmed's head.
"He arguably could be thinking about franchise cricket but we've given him an offer he can't refuse really - a chance to be involved in Test cricket as the pinnacle. If you can play this form, you can play anything."
That last bit is a principle Key swears by, and forms the basis of his work so far at the head of the English game, as far as on-field matters are concerned. Even from his days in the commentary box with Sky Sports and others, he has long held a view the old and new worlds can sit comfortably together, with a little give and take along the way.
"He's a wildcard pick," Key said. "You talk to the franchise owners and you come up with a plan so we are aligned. They want the same thing, which is Jofra Archer not getting injured again for a long period of time. It's handy that he can bowl four overs in two games for them, then go into the 50-over stuff, so he has competitive cricket and a build-up. The way the world works now, you have to work with these teams and all you have to be aligned and want the same thing, to make sure Jofra can play to his potential for as long as he can. The only way it works is if you all work together."
At 37, and having just retired from a playing career primarily with Sussex across 20 years as a professional, he is wired into the game, both with contacts and his understanding of an ever-evolving ecosystem. He has experience across the world, in a variety of domestic and franchise competitions. Beyond his undoubted personability, Key will lean on his rare nous.
"Things like shaping central contract decisions, all those things he'll be involved with, and he understands better than I do because he's played franchise cricket and knows what it's like to be a player, and the decisions they've got to make because these decisions are coming now for the players. They're not coming in five years' time. This is coming now. Which franchise do they want to play for? What format do they want to play? Do they want to play in that series or is it going to collide with something else? Luke Wright's across all of that."
Of course, a lot of this is a very English luxury, be it guaranteed interests in Test cricket from participants and punters, or simply the finances to ring-fence their assets to a point. Key acknowledges all of that, especially at a time when franchise competitions are only growing in number and pull.
"We are so lucky in English cricket but our summer doesn't get decimated by all of these leagues. You can see why Rahul Dravid said how they can't let their players play in these franchise leagues, because all these leagues would just decimate the Ranji Trophy.
"We're in a very fortunate position but we've got to realise that we've got to work these people and put ourselves in the player's shoes and think about 'what decision would I make here?' You've got to be fair and make sure everyone benefits. As expected, it's just come quicker than I thought."
Vithushan Ehantharajah is an associate editor at ESPNcricinfo