Imam, the 22-year-old nephew of Inzamam who made a century on his ODI debut against Sri Lanka late last year, was given a chance in the longer format of the game.
Fast bowler Wahab Riaz, 32, is omitted, with leg-spinner Yasir Shah injured.
"If we look at his numbers (of wickets) for the last two years, he has more wickets than any other bowler for Pakistan".
Sarfaraz Ahmed, Azhar Ali, Sami Aslam, Haris Sohail, Babar Azam, Fakhar Zaman, Inam-ul-Haq, Asad Shafiq, Usman Salahuddin, Shadab Khan, Muhammad Amir, Muhammad Abbas, Saad Ali, Hasan Ali, Faheem Ashraf and Rahat Ali.
Yasir has taken 164 wickets in 28 Test matches since making his debut in 2014 and has been instrumental in Pakistan's success - especially in the United Arab Emirates.
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Faheem Ashraf is another all-rounder included, who could prove handy in batting down the order.
Saad, 24, is the leading runscorer in the Quaid-Azam Trophy, Pakistan's domestic first-class competition.
Nine batsmen have been included in the squad to counter the hard playing position in England.
Chief Selector Haq thanked the Pakistan Cricket Board (PCB) for allowing the selectors to announce a 16-man squad instead of the customary 15 to allow for a strong bench.
Sporting a young side, including a number of players who would be looking to make their Test debuts, Pakistan will play a lone game against Ireland - Ireland's first - starting May 11, followed by two Tests against England.