The Frankfurter Rundschau called it a Schnapsidee, an idea that was so outlandish it could only have come about under the influence of alcohol. Rhein-Neckar-Zeitung described it as a “PR-gag”, an opportunity for one of the smaller Bundesliga clubs to get some nationwide and even worldwide publicity. And perhaps the scepticism was inevitable.

Hoffenheim, deep in relegation trouble, had just responded to the news that their hugely experienced manager, Huub Stevens, was leaving due to health problems by appointing the under-19s coach, Julian Nagelsmann, in his place. Nagelsmann’s inexperience was one thing but above all it was his age that stood out: he was 28.

His appointment, which came on 11 February, 2016, made him the youngest ever coach in the Bundesliga and he had not even finished his coaching qualifications. Five players in that Hoffenheim squad were older than him. The club were seven points adrift of safety. It had all the potential to end in tears. But it did not. In fact, his appointment has been an utter triumph. Hoffenheim avoided relegation at the end of the 2015-16 season and finished fourth the next, which is why the now 30-year-old is preparing to take on Liverpool in the Champions League play-off first leg on Tuesday.

And, as far as Hoffenheim were concerned, Nagelsmann’s appointment was never a gamble. As the club’s sporting director, Alexander Rosen, said at the unveiling: “Twenty-eight. It sounds brutal doesn’t it? But it is definitely not a PR stunt. We are completely convinced he will be a success. He is unbelievably mature and has already been coaching for 10 years.

“When you see him on the training pitch, when you see how he coaches the players and when you see, despite his age, the natural authority he possesses you realise that he really is something special.”

The new coach immediately impressed the players with his tactical knowledge and his confident demeanour. Nagelsmann’s plan was quietly to take players aside one at a time to point out a tactical issue every now and then rather than to act like a “loudmouth” who shouted at the whole squad in order to earn respect. It worked.

Nagelsmann is very mature for his age. He was forced to retire from playing at the age of 20 after persistent knee injuries and at around the same time his father died. At one point it was almost too much for him to deal with and he considered quitting football altogether.

“It was tough,” he said in an interview with the Frankfurter Allgemeine last year. “I had to grow up a lot quicker than a lot of others. I had already been living on my own since I was 15 – cooking for myself, doing all the shopping and so on – and then when my dad died I had to help my mum to sell the old house, find a new one. That’s a lot for a 20-year-old to take on. All that happened was extremely tough but it also taught me that there are things far more important in life than football and I think it helped me mature. In football we shouldn’t take ourselves too seriously. You just have to turn on the news on the TV in the evening to realise straight away how unimportant you are as a Bundesliga coach.”

Julian Nagelsmann watches his Hoffenheim players in training on Monday. Photograph: Ronald Wittek/EPA

Nagelsmann’s self-distance is a key component to his and Hoffenheim’s success and so is his constant desire to develop. He was helped along the way by a certain Thomas Tuchel, who was coach at Augsburg, the club Nagelsmann played for when he was forced to retire. Tuchel recognised what a good tactical mind Nagelsmann had and asked him to become his opposition scout.

“Tuchel saved me,” Nagelsmann told 11Freunde in 2013. “I was sick of football. I had sacrificed my whole youth for football and then, bang, overnight, it was all over. I didn’t want anything to do with football. But when he asked me to become opposition scout it was a win-win situation for everyone. I still had a contract with Augsburg and I could help him.”

Nagelsmann arrived at Hoffenheim in 2010 to become co-coach of the under-17 team. Two and a half years later, as a 25-year-old, he was suddenly promoted to assistant coach under Frank Kramer for the first team. He continued under Marco Kurz before returning to the under-19s until his big moment came in February 2016.

Nagelsmann – who admits he has an awful lot of energy, “sometimes too much” – says he has his best ideas in the bath in the morning. “I’m there in the bathroom with a ruler and a pencil thinking about ideas. I then draw my idea on a notepad. Later I look at whether that thought is even realistic, search for video clips and then, if I still think it is a good idea, I try it out on my team.”

Nagelsmann rarely chooses the same formation in consecutive games – in the 14 games that kept Hoffenheim up in 2015-16 they only lined up with the same tactic three times – and while he admits he can “exhaust” the players with his constant tinkering he also feels that it helps them develop.

“It is extremely important to keep players stimulated all the time, in training and in games,” he has said. “Football is actually pretty limited and there are only really four phases: When you have the ball yourself, when the opponent has the ball and when you win the ball or lose the ball. That is football, really, there isn’t more to it.”

“You can practice these four phases with thousands and thousands of exercises in training, with new rules, new formations and so on, but key is that the player is alert all the time and thinks about what he is doing. It is also important that the players are happy with what they are doing. True, they earn a lot of money but their existence can be quite repetitive and there for you have to vary trainings sessions and make sure they are enjoying what they are doing.”

The Hoffenheim manager’s tactical outlook is not dissimilar to that of his opposite number on Tuesday, Jürgen Klopp: pressing, gegenpressing and, as one paper described it last year, “chaos” in defence. Nagelsmann wants his players to force opponents into mistakes by crowding them out and putting pressure on them then hit them on the counterattack.

He also, in common with one of his role models, Pep Guardiola, does not want his players to tackle too much. “There are too many unknown quantities in tackles,” he said. “The ball can fall here, fall there, take a bounce, go out for a throw-in or end up at an opposition player’s feet. The referee can give a free-kick to us or to our opponent. Nah, I am not too keen on tackles. I much prefer us to win the ball by pressuring the opponent into a mistake, that we cut off his angles so that he makes a mistake when he tries to find a team-mate with a difficult pass. And then, when we get the ball, we need to be quick in order to be able to create a chance and then it’s better and the player is fresher when he hasn’t had to make five tackles before starting an attack.”

His training sessions are legendary. He is arguably the greatest innovator in the Bundesliga. Bild called him a “rule-revolutionary” and listed some of his thoughts: he wants to have one transfer window, from February until mid-May – he would like two time-outs per half to allow the manager to react to what is going on. He is also in favour of two, tennis-style, Hawk-Eye challenges per team per game at the behest of the manager while he wants more substitutions and has suggested five-minute sin-bins instead of yellow cards.

Nagelsmann simply never stops. Hoffenheim are one of only two clubs (Borussia Dortmund is the other) to use the Footbonaut training centre, where balls are hit to players at speed to improve their control and decision making, and this summer he erected a video wall at the club’s training ground.

A huge 6mx3m screen was placed on the halfway line and he then used four cameras, two of them from adjacent towers, to watch the training session. When he spots something, he stops training, rewinds and plays the offending clip on the giant screen. “I have an iPad in my hand that I can use to control the cameras. When I stop a situation, I have the opportunity to draw my solutions and suggestions for improvement, all from the iPad,” he explained.

Kevin Kurányi, the former Stuttgart, Schalke and Hoffenheim striker who played 52 times for Germany and who was one of the squad members who was older than Nagelsmann when he took over, once said: “I had a lot of great managers as a player but to be able to experience him was really interesting. I had never experienced a training session like his, a training session that was so demanding for the head.

“But I really liked it. When you didn’t concentrate completely for the full two hours by the exercises you had no chance. Every player must think about what they are doing the whole time, otherwise you are out and mocked by your team-mates. The success of Hoffenheim is first and foremost because of him. I think that one day he will coach Bayern Munich.”

Nagelsmann was voted 2016-17 Bundesliga coach of the year by the league’s players, way ahead of RB Leipzig’s Ralph Hasenhüttl and Bayern’s Carlo Ancelotti, and this summer extended his contract to 2021. He also continued to freshen things up by letting his players decide the club captain – they kept faith in Eugen Polanski – as well as this season’s targets.

This week all the focus is on the Liverpool game and the upcoming Bundesliga season. Hoffenheim have lost two of their best players, Niklas Süle and Sebastian Rudy, to Bayern Munich for a combined £19.3m and have bought modestly with the £3.1m capture of Borussia Mönchengladbach’s Nico Schulz the most expensive signing so far. The former Arsenal player Serge Gnabry has joined on loan from Bayern.

The new season is likely to be the most exciting in Nagelsmann’s short spell in charge, but perhaps also the most difficult. Just like he wants it.