Matt Roller is an assistant editor at ESPNcricinfo. @mroller98
SL v AFG (1)
PSL 2024 (3)
NZ v AUS (1)
WCL 2 (1)
CWC Play-off (3)
WI 4-Day (4)
Rehan became the youngest man to take a five-wicket haul on Test debut on Tuesday, running through Pakistan's middle and lower order to return figures on 5 for 48 and swing the Karachi Test England's way.
He played 14 T20 Blast games for Leicestershire last summer and five times for Southern Brave in the Hundred, and is available at a base price of INR 40 lakh (£40,000 approx.) in Friday's auction. There is rarely much demand for overseas spinners at the auction, but Rehan's low price point and huge potential could convince a franchise to take a punt on him.
"It would be great," McCullum, who played in the first 11 IPL seasons and recently coached Kolkata Knight Riders, told the BBC's Test Match Special. "It would be awesome if he did. Why not? Why not get an opportunity to go and play under different coaches and different captains and rub shoulders with different players and pick up those experiences? What 18-year-old kid anywhere else in world cricket is going to get those chances? I think we should encourage it."
Mo Bobat, the ECB's performance director, said in October that Rehan had been "inundated" with offers from franchise leagues for the 2022-23 season and that "English cricket has a real responsibility to manage him carefully". He has already pulled out of one tournament - the Pakistan Junior League - this winter and has a contract with Andy Flower's Gulf Giants in the inaugural ILT20 in the UAE in January and February.
His availability for that tournament could be curtailed if England pick him for any of their ODI series in South Africa in late January, their Test tour to New Zealand in February or the Lions tour to Sri Lanka, but McCullum said he would "encourage him to play as much franchise cricket as he can".
McCullum said: "He's got a real future. He's a guy who has been on the radar of English cricket for a little while. It took some courage from Stokesy [Ben Stokes] and a few people to thrust him into this opportunity. The more we're prepared to do that, the more we can discover how talented people are and how much they can grow on the big stage as well.
"The important thing for Rehan is that we continue to look after him. He won't be in every squad and I'll encourage him to play as much franchise cricket as he can, and get as many experiences around the globe and play with different players, under different coaches and in different conditions.
Ben Stokes hands a stump to Rehan Ahmed after England's victory in Karachi•Matthew Lewis/Getty Images
"His skill is not a problem, his attitude is absolutely brilliant. We've just got to get as much experience into him as possible. That's not just at Test level but in leagues as well. I couldn't be happier for him: it will give other kids around England some motivation that if you work hard enough and you've got skill and the game is your passion, then you can actually achieve it, maybe sooner than you thought."
If Rehan is not picked up in Friday's auction, he will spend the first two months of the English season with his county, Leicestershire. He played only three County Championship games out of a possible 14 last year, and was seen as the second spinner behind the slow left-armer Callum Parkinson.
Asked if he would like to see Rehan given more opportunities in the Championship, McCullum told Sky Sports: "You can't force anyone to do anything. But I think maybe a few people might have realised just how talented he is and what a difference he can make in the game.
"I'm pretty relaxed, to be honest. [He could] go and play some franchise cricket around the world, get some exposure to different conditions and try and play as much cricket as you possibly can, and get as many experiences as you possibly can in the shortest period of time. We'll end up benefiting from it."
Stokes said that Rehan's emergence was "seriously exciting" for English cricket. "One thing I was very impressed with is that the more he got into the game, the more confident he felt in himself to start talking to me about fields and what he wanted the batsmen to try and do," he told Sky.
"Legspinners are absolute gems. They can change the game and we've seen that in that spell yesterday afternoon when he came in and bowled us to victory. For an 18-year-old to have the ability that he does, being nowhere near the finished article in my opinion, is seriously exciting."
Rehan was watched by his father, Naeem, throughout the Karachi Test, whom England invited into their pre-match huddle when Nasser Hussain presented Rehan's cap. "That cap presentation was absolutely brilliant," McCullum said. "There were a few tears flowing around the dressing room after that.
"Even a couple of days deep into it, the boys were still talking about how amazing that moment was and sometimes reminds you that you should never take it for granted. Rehan is just an amazing story. He's a guy who loves the game so much, his family love the game so much.
"He's clearly got a huge amount of talent and ability. And we've just got to allow him to be able to go up at his own pace. We'll give him the opportunities when they come but also we'll look after him when we need to. He couldn't have been any more impressive than what he's been so far."