England skipper Ben Stokes has continued his bold captaincy approach by naming the uncapped 18-year-old in his team as the tourists go in search of a series whitewash in Karachi.
Ahmed earned a call-up to the senior set-up after impressing for Leicestershire last season and then England Lions during a warm-up camp in the United Arab Emirates.
Ahmed only made his English County Championship debut for Leicestershire in May, but Stokes and head coach Brendon McCullum have insisted throughout this series that they would have no qualms over picking him.
"I think it's a good opportunity for us to not only look at him but for him to come in and experience what it's like to play international cricket," Stokes said.
"We obviously brought him into the squad to try and get him amongst us and experience what it's like being in this cricket team and we've been very impressed with what we've seen.
"He's got a lot of skill with both bat and ball, so it's great to be able to bring someone with the talent and the excitement that he has into the squad and see what he's got this week."
Ahmed attracted media attention at just 11 years of age when he dismissed Stokes and Alastair Cook in the nets having been invited to bowl at the national team.
He also received high praise from former Australia spinner Shane Warne, who when he saw him bowling in the nets at Lord's as a youngster said in a video on Twitter: "That's awesome man, really really good, I'm going to keep a close eye on you. I think we're going to be commentating on you really soon.
"I think you'll be playing first-class cricket by the age of 15."
On Saturday, Ahmed will beat the record held by Brian Close, who was 18 years and 149 days old when he first represented the country in 1949.
Ahmed has already impressed in domestic cricket despite only playing three first-class games, hitting a century against Derbyshire in September, and Stokes was full of praise for his ability with bat and ball.
"I think being at such a young age, it's great to see someone who has so much freedom in what they do and the way that he bats, he likes to get on with it and show what he's about and almost - not show off - but show what he can do," he said.
"He's got a vast array of shots. And obviously a wrist-spinner is great to be able to have in your team, especially in these conditions.
"We've taken a look at the wicket as well which looks very dry, I personally think it might spin a bit more than it did in the last Test match but I'm not great at looking at pitches.
"It's exciting times for Rehan especially, and he was very excited when we gave him the nod that he was going to play last night."
Ahmed replaces all-rounder Will Jacks while England's other change sees wicketkeeper-batter Ben Foakes return in place of seamer James Anderson.