Pakistan opener Salman Butt was admitted to hospital after complaining of severe stomach pain after the first day of the first Test against South Africa.
The home side's vice-captain fielded for the opening day of the Test match in Karachi but was taken ill during the night.
After complaining of severe stomach pains, Butt was taken to hospital for tests, where he is still under observation.
"At around 3am today, Butt had a severe pain in his stomach and we took him to the hospital," Pakistan team manager Talat Ali said.
"The doctors at Agha Khan Hospital feared that Butt might be suffering from gastroenteritis.
"We don't know when the doctors will release him."
Yasir Hameed took the field on the second day of the Test in place of Butt, with South Africa resuming their first innings on 294-3.
Ali said clarification would be sought from the match referee as to whether Butt could open the batting should he be cleared to return to the game.