The three-time winners and last season's semi-finalists get their campaign underway against Surrey on May 30 at the Utilita Bowl
Naveen-ul-Haq will not be available to Hampshire Hawks for his proposed spell during the T20 Blast.
The seamer was due to arrive after representing Afghanistan at the Men's T20 World Cup for the final six group matches, replacing Australian Michael Neser, and would have been available for September's knockout stage.
A statement confirming the news cited "unavoidable personal circumstances" for the reason behind the withdrawal.
Related: "I've potentially got what they're looking for" - John Turner embracing England ambition
"Advanced discussions" are underway in the process to recruit a replacement. The three-time champions get their campaign underway on May 30 against Surrey at the Utilita Bowl.
Wicketkeeper-opener Ben McDermott is confirmed to return for a third straight campaign for the duration of the competition.
Michael Neser is due to join up for eight group matches (Hagen Hopkins/Getty Images)
The signing of Naveen, even for a county with Hampshire's white-ball reputation, was a coup when it was announced in February. Across three summers at Leicestershire he took 54 wickets in 33 matches and was a seismic presence bowling at both ends of the innings.
Currently playing in the Indian Premier League for Lucknow Super Giants, he has a dozen wickets in nine matches.
The news is offset by John Turner's return to full fitness. The 23-year-old has been limited to second XI outings this summer due to a knee injury but, now full fit and overlooked for selection in England's T20 World Cup squad, he is set to have a full role for the south coast club.
In his maiden campaign, Turner took 21 wickets in 11 matches as Hawks reached a record 10th Finals Day.
Join The Cricketer's brand new Whatsapp channel for the latest breaking news, comment and features - click here to become a member
Welcome to www.thecricketer.com - the online home of the world’s oldest cricket magazine. Breaking news, interviews, opinion and cricket goodness from every corner of our beautiful sport, from village green to national arena.