The 23-year-old suffered the blow during the 67-run victory in Karachi and will be replaced by Abid Ali for the final game of the series
Imam-ul-Haq will miss Pakistan's concluding one-day international against Sri Lanka with a hand injury.
The 23-year-old split the webbing in his left hand while fielding at point during the Sri Lanka chase in the second ODI in Karachi.
He immediately left the field after deflecting a drive from Sadeera Samarawickrama and did not return for the rest of the chase.
Abid Ali will replace him for the third ODI, with Pakistan looking to wrap up the series which they lead 1-0 after the 67-run success on Monday.
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"Imam-ul-Haq is not available for selection due to an injury in his left hand, which he sustained while fielding in the second ODI against Sri Lanka,” a PCB statement read. "Opening-batsman Abid Ali replaces him in the side."
Imam had earlier hit 31 from 41 balls as Pakistan reached 305 for 7 in their 50 overs.
Despite Imam's departure, Usman Shinwari took 5-51 as the tourists were bowled out for 238. Following the series Pakistan and Sri Lanka face-off in three T20s starting in Lahore on October 5.
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