Who is former Schalke coach David Wagner?

How did Wagner go from being a previously unknown member of Jürgen Klopp's coaching staff at Borussia Dortmund to crossing the Ruhr Valley divide to take charge of Schalke? Allow bundesliga.com to fill in the gaps.

"Somebody, I don't know how it happened, put us together in a room and that was the start of a lifelong friendship," Klopp said in a Sky Sports documentary. "It's like family, so we consider each other brothers and it also feels like we are because we've known each other so long."

On the field, Wagner enjoyed a relatively successful spell in the second division at Mainz, scoring 19 times in his four years at the club. "He wasn't very consistent, even if he does not want to hear it," said a typically forthright Klopp of his former team-mate's ability. "He was a big talent, but not every day. He was very a young player when he came from Eintracht Frankfurt to Mainz, a very skilled boy, very quick, a good striker."

Pioneer at Huddersfield

With a burgeoning reputation in the dugout and the coaching foundations duly laid, it was no surprise that Wagner was rumoured to be joining Klopp at Liverpool in late 2015. Tempting as that may have been, he opted to forge his own path and although he did move to the north of England, it was to Huddersfield instead.

At the time the Terriers were in the midst of a relegation battle in the Championship - England's second tier - and after sacking Chris Powell, they were looking to revolutionise their working model.

"The first phone call I received was from [Huddersfield head of football] Stuart [Webber] and at the end of our conversation he said the club had to find a new way, that everybody wanted to go in a new direction, and were excited about what we did in Dortmund and how we tried to play," Wagner explained.

"He asked me: 'Is it possible to do this in England?' I said: 'Why not?' It's possible to play this way anywhere in the world if you have open-minded players and an open-minded club."

Wagner found fertile ground in Huddersfield, and, after overhauling the team's style, he led them away from the relegation zone by the end of the campaign. In 2016/17, his first full season at the club, Wagner took a team with one of the smallest budgets in the English Championship and achieved promotion to the Premier League via the play-offs.

A one-off success, perhaps? Far from it. Wagner diligently built on that achievement in 2017/18, a campaign in which the Terriers were initially widely considered to be relegation favourites, before proving their doubters wrong and securing safety with a game to spare.

Not only that, but the manner in which they did so was also remarkable, recording draws away to Chelsea and Manchester City - Premier League champions past and present - in two of their last three fixtures.

The 2018/19 campaign proved tougher, and Wagner and Huddersfield went their separate ways in January with the team struggling against relegation, a fate they would not escape.

Schalke, however, had seen enough to convince them that the former USA international, who scored twice in 29 Bundesliga appearances for the Gelsenkirchen outfit between 1995 and '97, was the man to reinvigorate their side. Again, Klopp played a part in helping Wagner make his decision to return to the German top-flight.

"Honestly speaking, if you had asked after I left Huddersfield about where I expected to work next, I would've said England," Wagner told Sport1. "But Schalke are just one of a few clubs in the Bundesliga that make it worthwhile to come back from England. From that moment on, I didn't think about the Premier League.

"Kloppo told me that Schalke totally suits me. It's a club with high emotions, with passion and power. Now we'll see whether or not he was right."

Things started superbly for Wagner's Schalke, who - following a draw and defeat in their opening two matchdays - won four on the spin from Matchday 3-6. That included a 3-1 win at early leaders RB Leipzig, that inflicted a first defeat of the season for Julian Nagelsmann's side.

Watch: David Wagner's tactical revolution at Schalke

Suddenly, Schalke had gone from relegation battlers to European contenders as they ended the midway point of 2019/20 in fifth-place. Unfortunately for club and coach, the league's coronavirus-enforced break seemed to throw them off course and Schalke closed out the campaign with a 16-game winless run as they finished 12th.

The dubious club record extended to 18 matches with two defeats to open 2020/21, including an opening day 8-0 loss at champions Bayern Munich. The second loss - at home to Werder Bremen - proved the nail in the coffin for Wagner's time at Schalke but, given his previous pedigree from the dugout, the 49-year-old will be back orchestrating touchline proceedings in no time.