Celtic manager Martin O'Neill remains in hospital after undergoing a "small procedure", the club have announced.
There are no details about the nature of O'Neill's operation, with the 74-year-old expected to be released in the "next day or two".
O'Neill signed a one-year deal in June to become permanent manager after two interim spells last season culminated in a Scottish Premiership and Scottish Cup double.
The Northern Irishman was in the dugout on Monday as Celtic started the new Premiership campaign with a 1-0 home win over Dundee.
"In response to speculation, we can confirm that Martin O'Neill has undergone a small procedure in hospital," Celtic posted on social media.
"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."
Having not managed since a spell with Nottingham Forest in 2019, O'Neill made a shock return to Celtic last October - 20 years after his hugely successful first spell in charge - following Brendan Rodgers' acrimonious resignation.
The former Leicester City, Aston Villa and Republic of Ireland boss then handed the reins to Wilfried Nancy in December, but the Frenchman was sacked after just eight games, paving the way for O'Neill to return again.
And after winning his eighth and ninth major honours with the club, he agreed to take on the job full-time, agreeing a one-year contract with the option for a further year after talks with Celtic's major shareholder Dermot Desmond.
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