Ronny Deila is to quit as Celtic manager at the end of the season, with the Premiership leaders announcing it was the Norwegian's decision to leave.
Neil Lennon, who Deila succeeded in 2014, has already said he would welcome a return to the Scottish champions.
News of Deila's exit comes three days after a Scottish Cup semi-final defeat by city rivals Rangers at Hampden Park.
"There have been some disappointments and times when we have not achieved what we had hoped for," he said.
"I'm realistic and honest enough to admit that."
Lennon is a free agent after leaving English Championship side Bolton Wanderers but is not the only potential candidate.
The move was on the cards after Celtic were dumped out of the Scottish Cup by arch rivals rangers at the weekend