Pakistan leg-spinner Shadab Khan is out of next month's one-day series in England because of a virus, raising doubt over his World Cup participation.
The highly-rated 20-year-old has taken 47 wickets in 34 ODIs but following blood tests, doctors have advised him to rest for four weeks.
Yasir Shah will replace him for the five-match series against England, beginning on 8 May.
Pakistan's opening World Cup match is against West Indies on 31 May.
The Pakistan Cricket Board said Shadab, the only specialist spinner in their 15 for the World Cup, will meet with specialists in England to help him fully recover for the tournament.
Teams must finalise their World Cup squads by 22 May.
Fast bowler Mohammad Amir and big-hitting Asif Ali are in the squad for the England series, but were not in the provisional World Cup squad.