Deila, who is now managing back in Norway with Valerenga, took over at Celtic in 2014 from Neil Lennon and was faced with an ageing squad in need of fresh blood.
Speaking to The Times, he said: “Neil Lennon’s team had been successful but it was getting older, it was past its prime, and my job was to bring young players in and develop them. That was why I got the job — the board believed I could develop a younger team.
“So I brought in guys like Kris Ajer, Ryan Christie, Stuart Armstrong, Gary Mackay-Steven and Dedryck Boyata. Plus, I brought Callum McGregor and Tom Rogic into the first team. Both Callum and Tom were at Celtic when I arrived but they weren’t really involved in the top team. Callum had been away on loan but I wanted to put him in the team.
“It was my job to make Celtic younger as well as successful. It was all part of the challenge. My job was to make it work.”
Deila had a successful first season in Scotland, winning the League Cup and Scottish Premiership title. He was unable to lead the club to a treble however, losing in controversial circumstances to Inverness Caledonian Thistle in the Scottish Cup semi final. With Celtic already ahead, Josh Meeking’s handled a goal-bound Leigh Griffiths effort on the line. Referee Steven McLean missed it and, after Craig Gordon was sent off in the second half, the Highland side would go onto win in extra-time.
Deila still rues missing out on his best chance at clinching a domestic clean sweep. “It was very painful, not getting that treble. There were seven referees at that match at Hampden and they all missed that handball. It was very tough to take. You don’t know what would have happened in the final but missing out on the treble in this way was painful.
“We had a very strong team in that 2014-15 season. Once we got going we were very hard to beat. The team didn’t know how to lose, it didn’t think about losing. But we were denied that treble and it is history now.”
Deila left Celtic after only picking up one trophy during the 2015-16 season - the Premiership title - in what was widely regarded as a disappointing second year in charge. Defeats to Ross County in the League Cup and Rangers in the Scottish Cup soured what had been a promising start to his time at Parkhead.
Brendan Rodgers replaced him that summer and oversaw a marked improvement in performances and results, leading the club to the treble as Celtic went the domestic season unbeaten. Despite using many of the same players who had under-performed under Deila, the Norwegian does not think it reflects badly on him.
“Like I said, my job was to bring in young players which I did — Ajer, Armstrong, Christie, Mackay-Steven — and to build a young team. It takes time for young players to develop and now I think you can see how good many of these players are.
“I think Brendan Rodgers is doing the same. I look at the job he is doing, in developing players, and I think he has been fantastic. To win six [domestic] trophies in a row — and it might be seven this weekend — is unbelievable. I am very impressed.”
Deila also believes that the experience at Celtic, and the intense scrutiny that it brought, has improved him as a manager.
“What I learned at Celtic, you couldn’t read in any book. Here in Norway, some coaches talk of the pressure we are under. I can guarantee you, that pressure is a small breeze compared to the pressure of managing Celtic.
“I learned how to cope with that pressure. How to play 60 games in a season. How to cope with having to win every weekend. How to [adjust] to a new culture and language. How Scottish football was thinking, and the mentality of the game in Britain. I could tell you 100 things I learnt about myself and football if you gave me the time.
“If I ever get the chance to manage again, in Britain or somewhere else in Europe, I think I will show that I am better for the experience I had at Celtic, and that I can handle it. I’m still only 43. There are some nights when I think, ‘I’m stopping this, I’m getting out of this job’, but most of the time I still love being a manager. I might still have another 20 years in this job.”