Ronny Deila reckons the stress and pressure of managing Celtic was too big for him....but insists he could take charge of Real Madrid now because of how much he learned at the Hoops.
The Norwegian is now in charge of Club Brugge after he was controversially head-hunted last season from Belgian rivals Standard Liege.
Deila won two league titles and a League Cup during his two-year spell in Glasgow before quitting in 2016 and then went on to win the MLS Cup with New York City.
And he's revealed how much of a toll it took on him. He said: "Few people can imagine how enormous the pressure is at Celtic.
"It is one of the largest clubs in the world. And I came from Strømsgodset, a small club at European level, comparable to, say, Eupen here in Belgium, where there are only 7 to 8,000 supporters in the stands.
"At Celtic there were suddenly 60,000 and the club has a total of 10 million fans worldwide.
"The city of Glasgow is crazy about football. You have to win there. I wasn't ready for it. I simply couldn't prepare for something like that.
"Mentally it was challenging, to the extent that the fun was gone at the end. I was up. Then you have no chance.
"But I would absolutely do it again. I am very proud of my progress there. I built a team full of young players who were virtually unbeatable in the years that followed.
"At the time it was tough. The step was too big. It was like going from first grade to sixth grade.
"It will never be as hard as it was then. But because I have now acquired the necessary knowledge. I now even dare to say that I could manage any team...even Real Madrid.
"Because of Celtic I feel I am up for any challenge. "
And Deila revealed the method in which he turned around Liverpool defender Virgin van Dijk's career after he joined Celtic a year before him.
"I immediately saw he had everything for the top so I started working intensively with him," he continued.
"He trained poorly because it was too easy for him in Scotland. And as a result, he was not fit.
"Under me, Virgil van Dijk lost eight kilos of fat. From then on he was a machine."
