On May 25, 2013, a few hours before leading out Borussia Dortmund in the Champions League final, Jurgen Klopp sent a text message to an old friend. "Without you," it read, "I wouldn't be here today, in London, at Wembley."

The recipient was Wolfgang Frank, Klopp's former coach and mentor at Mainz. Employing Frank's then-revolutionary zonal marking and high-paced pressing tactics, Klopp had managed to bring his master's life-work to fruition and got the low-key 2.Bundesliga club promoted to the top flight for the first time in their history, and a few years later moved on to win two championships with Dortmund.

Through his spectacular successes, German football at last understood the importance of Frank, an introverted visionary who had been simply too ahead of his time and too stubborn to find wide-scale recognition during his time on the bench. He tragically passed away aged 62, only four months after receiving Klopp's message. A malignant brain tumour claimed his life.

In just over three weeks' time, on the afternoon of May 26, Klopp might well send a similarly grateful message to another old friend who's no longer by his side -- momentarily, at least. Irrespective of the "personal reasons" that have seen his lieutenant Zeljko Buvac leave his post until the end of the season (as Liverpool's official statement has it) last week, there won't be any doubt in the Liverpool manager's mind about the enormous debt he owes to the 56-year-old Bosnian.

For a mostly successful 17 years, Buvac had been "a brother in spirit," as Klopp put it, his sounding board and loyal confidant. By the 50-year-old's own admission, appointing his former Mainz teammate as his assistant ahead of his first full season on the touchline at Mainz's former Bruchweg stadium home in the summer of 2001 had been the best transfer decision he ever made. Without him, rookie coach Klopp might have never lasted a whole campaign in the unforgiving environs of 2.Bundesliga, let alone won promotion with Mainz and three trophies with Dortmund a few years later.

Zeljko Buvac has been Jurgen Klopp's assistant at Mainz, Dortmund and Liverpool Getty

The two of them had toiled for three seasons together from 1992 to 1995 at perennially relegation-threatened Mainz. Klopp, a lanky striker from a small town in the Black Forest, and the skilful midfielder who moved to Germany after the fall of the Iron Curtain in 1991 had little in common, apart from a love of tactics. They would spend hours on end debating strategy and playing styles, trying to find novel ideas that would lift a squad of below-average quality out of the quagmire.

Jan Doehling, editor at Mainz-based public broadcaster ZDF recalled that that the unlikely duo made a pact: "Whoever became a coach and had the first big job would bring in the other one."

After finishing his career as a player in 1998, Buvac tried his hand at managing at Borussia Neunkirchen in the third and fourth division. In another life, Klopp might have become his assistant. But it wouldn't have been a natural fit, according to Doehling: "Klopp is the salesman, the face of the operation. Buvac is the man for the details behind the scenes, the guy who never talks."

Christian Heidel, then the Mainz sporting director, was hugely impressed with Klopp's decision to bring in his former teammate. "It showed that Klopp saw the bigger picture, he didn't just trust his instincts," he said.

It spoke of humility, too. Roping in Buvac amounted to an admission that the then-rookie coach, aged 34, needed the help of a more experienced, slightly older colleague. Immediately after he had been promoted from player to caretaker manager to save from the drop in the spring of 2001, Klopp had still "pretended to know everything already" while harbouring "a thousand questions with no one ask." Buvac had the answers.

"He was my first choice, and he would have been my second and third choice, too," Klopp later said. "I think the biggest strength of strong people is to put people around you who are stronger in specific areas than yourself."

Some reports have miscast the Bosnian as the brains behind Klopp's trademark gegenpressing style. In truth, it was Frank who had laid the foundations for that high-energy chasing of the ball. Buvac's collaborative input was nevertheless key.

At Mainz, he set up obstacle courses with poles and benches that simulated the random nature of midfield ricochets and ensured that players trained for the hugely demanding pressing style without getting bored in the process. He could read an opponent's game at a time when video analysis was still in its infancy, implement minor adjustments that made a big difference and instantly win the respect of the dressing room by showing off his considerable technique in training.

Jonathan Brady / PA Wire/Press Association Images

Dortmund defender Neven Subotic revealed how pleased the team were to see Buvac take charge of two Champions League matches in 2013-14, in the wake of Klopp's suspension for a touchline bust-up with a referee against Napoli. Buvac himself did not enjoy the limelight, however.

Even though Klopp spoke of a "telepathic understanding" between himself and his assistant, their relationship was combustible at times. A fellow friend said they often bickered "like a married couple," falling out over footballing questions before making up again in due course.

"There were a lot slammed doors, a lot of cursing," Heidel said. "But in the end, they were in each other's arms again."

We are yet to learn whether Buvac's departure last week was motivated by another one of those disagreements -- and if so, whether there remains a chance for reconciliation. In the short term, the repercussions of his absence will be negligible. Klopp's team are sufficiently well-drilled, tactically, to make do without Buvac for the last three games of the season. In addition, his second assistant coach, Peter "Pete" Krawietz, who has been at Klopp's side for as long as Buvac, has become increasingly influential in recent months and can fill the void.

In case the trial separation from a man former Mainz president Harald Strutz called Klopp's "identical twin" becomes permanent, the Liverpool boss will find it nigh impossible to identify a like-for-like replacement. But the need to draft in a new member of the "band," as Klopp has described his backroom staff, also offers a chance to come up with some new tunes.

Since their arrival at Anfield two-and-a-half years ago, Klopp, Buvac and Krawietz have worked hard behind the scenes to make their team less reliant on the transition phase and to develop a more refined possession game that's needed to break down defensive opponents.

More progress might be made in that respect if a coach with a different, more combination-centric footballing background was to added to the fold. Klopp's biggest strength as a manager has been his willingness to learn from others and take on board outside influences. Losing Buvac, "the brain," as Klopp has described him, could be a catalyst for thinking things differently the future.