He was 28 years old at the time, and the team sponsored by software magnate Dietmar Hopp — 17th place in the table, five points adrift of the relegation play-off spot — were well on their way to a first abstieg descent from the top flight.
The idea has turned out a pretty good one though. The rather straightforward win against a Leverkusen team was depleted within six minutes at the BayArena — TSG old boy Kevin Volland brought down Kerem Demirbay with an ill-timed tackle outside the box — lifted unbeaten Hoffenheim into fourth spot in the table, four points behind leaders Bayern.
Their unexpected position chimes with the main story of the season so far, the surprising rise up the table by lesser lights such as newly-promoted RB Leipzig second , Hertha third , Köln fifth , Frankfurt seventh and Freiburg eighth at the expense of the establishment.
At this level, the quality of the players at your disposal will ensure that you play well within a good tactical set-up — if the psychological condition is right.
Nagelsmann was forced to the sidelines early on. At Hoffenheim, he worked as assistant coach of the first team before taking charge of the second side and finishing a degree in sport science.
But the plan almost looked like ending before it had reached its peak. Words with Bayern chairman Karl-Heinz Rummenigge, sporting director Matthias Sammer and even Pep Guardiola, one of his heroes and then-Bayern manager, were not enough to make Nagelsmann consider leaving Hoffenheim.
A born-and-bred Bavarian, at the age of just 20, his aspirations of being a professional footballer were already over. That manager turned out to be Thomas Tuchel.
Tuchel, the now-manager of free-flowing Borussia Dortmund and eventual opponent, instructed Nagelsmann to scout their opposition.
His stay in Munich would last two years, before leaving Bavaria to join Sinsheim club TSG Hoffenheim, to take up the same role — this time with a more long-term vision.
Hoffenheim were, at this point, a stable Bundesliga side under former Stuttgart and Schalke manager Ralf Rangnick, after a rapid rise to the top.
The club had just recently sacked Markus Babbel as head coach, and Frank Kramer, then in charge of Hoffenheim II, took caretaker charge of the first team with the young Nagelsmann, aged 25, as one of his assistants.
He stated, when asked about his appointment: I did not think he [Kramer] was being serious! The Juniors would also win the German Junior Cup without conceding a goal and a win in the final over Hannover Admirers would be gained and his philosophy would begin to unfold.
Maybe this stems from his psychological strength he had to follow the coaching path after wanting rid of football following his injury.
His stock was growing rapidly. Nagelsmann once again began the season in charge of the unders, expecting another strong season and that was the case.
However, things would eventually shape up rather differently. Huub Stevens, an experienced Dutch coach, was brought in to replace him on the same day, until the end of the season, with the hope to steadying the ship ahead of the new campaign.
In every case, he was always only going to be a short-term solution. His remarkable work was about to bear fruit years early. It was given to him by former Hoffenheim and Germany goalkeeper Tim Wiese as Nagelsmann was making a name for himself after the defender's career was cut short by persistent knee problems.
Aged 20, he was forced to end a promising career while in Augsburg's second team, coached by Thomas Tuchel, who is now manager at Borussia Dortmund.
Nagelsmann then worked under Tuchel before moving to Hoffenheim via coaching Munich's Unders. He then appeared on Bayern Munich's radar after masterminding Hoffenheim's German Under Championship triumph in , but he opted to stay at the Rhein-Neckar-Arena.
And his appointment as head coach came six years after arriving at the Sinsheim-based club. Hoffenheim had two wins from 20 games when Nagelsmann took charge nine months ago.
They were next to bottom in the table and seven points from guaranteed safety. Local newspaper Rhein-Neckar-Zeitung labelled his appointment a public relations stunt.
He had already been here six years so we knew what he was able to do. Hoffenheim won seven of the remaining 14 games to finish one point above the relegation play-off spot.
Hoffenheim had announced in October that the youth coach would take over as manager in the summer of , provided they stayed up.
However, Nagelsmann was fast-tracked into the job with three months of the season remaining when experienced boss Hubb Stevens resigned because of a heart complaint.
And Nagelsmann's sense of technical know-how and ability to man-manage has converted critics into admirers. They were bottom of the table during the winter break and everybody thought they would go down as they had no match plan.
With Hoffenheim's place in the Bundesliga secured, Nagelsmann got to work on rebuilding the squad in the summer. Hoffenheim have scored in all but two of their 24 league games under Nagelsmann and even took the lead at champions Bayern Munich, managed by three-time Champions League winner Carlo Ancelotti.
Every coach dreams of being able to celebrate only a fraction of his achievements," added Nagelsmann, who was raised in the Bavarian village of Issing.
Since their young coach took charge, Hoffenheim have taken 43 points in 24 matches. But Rosen warned there will be times in the future when it may not be so rosy.
We have to be careful we give Julian the air to breathe when things are not going well.