SunSport Online revealed that the ex-Bournemouth gaffer has drastically retired from work.
Celtic then confirmed the news – club sources claiming they were in advanced talks with a new target.
Former Australia boss Postecoglou is said to be that man – and has come out of nowhere to be installed as the bookies favorite.
More SunSport may reveal that the Yokohama F Marinos boss is well known to members of the Celtic hierarchy.
The Japanese club is part of the City Group, which has developed close ties with the Hoops in recent years.
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Postecoglou, 55, previously led the Socceroos at the 2014 World Cup and won the Asian Cup and domestic titles in Australia and Japan.
He is a left-field candidate but has a good reputation as a coach, plays attacking football and would jump at the chance to move to Celtic.
Former Borussia Dortmund boss Lucien Favre also has admirers on the Hoops board and remains a free agent.
The 63-year-old showed his interest in working at Celtic months ago and is still out of work after leaving Dortmund in December.
The Swiss coach is well rated after a great job at Borussia Mönchengladbach and Nice.
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Like Postecoglou, however, he has no experience of the British game.
Former Celtic player Roy Keane also remains interested in a return to Parkhead.
The Hoops chiefs now face a race against time to name a new man, with the preseason just three weeks away.
Celtic believed the deal with Howe would be made when Bournemouth’s season ended in a play-off loss to Brentford last weekend.
But further talks were stalled and on Thursday Howe informed Celtic that his behind-the-scenes team would not be available and that he was out.
However, the Parkhead club have revealed that they are in advanced talks with someone, with the aim of appointing a new manager “very soon”.
A statement from Hoops said: “We can now confirm that Eddie will not join the club, for reasons beyond his control and that of Celtic.
“We wish Eddie every success for the future.
“As part of the ongoing process, we have engaged with a number of candidates. “
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