Celtic have announced that Martin O'Neill is recovering after undergoing "a small procedure" in hospital.
The 74-year-old signed a one-year deal in June to become permanent Celtic boss after two interim spells last season ended in success, with a Scottish Premiership and Scottish Cup double.
He was in the dugout for the 1-0 win over Dundee in the league opener at Parkhead on Monday night.
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A statement on Celtic's official website called for privacy as the Northern Irishman recuperates ahead of the trip to Kilmarnock on Sunday.
"In response to speculation, we can confirm that Martin O'Neill has undergone a small procedure in hospital," the statement said.
"We would envisage Martin being released from hospital in the next day or two and on behalf of Martin, we would like to thank supporters for their kind wishes.
"We would ask that the privacy of Martin and his family is respected."