Martin O'Neill remains in hospital after undergoing "a small procedure", Celtic have confirmed.
The club said the 74-year-old is expected to be released in "the next day or two".
While no details were released regarding the procedure, Celtic added they "would like to thank supporters for their kind wishes".
O'Neill was named permanent boss this summer after guiding Celtic to a Premiership and Scottish Cup double last season.
He was in the dugout on Monday as Celtic kicked off their league title defence with a 1-0 win over Dundee at Parkhead.
Celtic are next in action on Sunday as they visit Kilmarnock, live on Sky Sports.
Celtic statement in full
"In response to speculation, we can confirm that Martin O'Neill has undergone a small procedure in hospital.
"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."
