Guardiola is now facing headache in central midfield after injuries to Thiago Alcantara and Bastian Schweinsteiger

New biography on Pep Guardiola claims he was made promise by Bayern that Kroos would never be sold

Bayern Munich have broken promises they made to Pep Guardiola when persuading him to join the club, a soon-to-be-released biography of the Spanish coach claims.

Written by Spanish journalist Marti Perarnau, it has emerged that one of Guardiola's conditions for joining Bayern was that Toni Kroos would not be sold 'under any circumstances'. Kroos was sold to Real Madrid for £20million this summer.

Perarnau's book Herr Guardiola has been written on the back of several months which the journalist spent shadowing Guardiola in Munich. It will be released in German at the end of this month.

VIDEO Scroll down to watch Toni Kroos embarass Ronaldo in Real Madrid training

Gloomy: Bayern Munich boss Pep Guardiola watches the German Super Cup defeat against Borussia Dortmund

In pre-release extracts, Perarnau describes details of the negotiations which brought Guardiola to Bayern. Having been approached, and offered huge sums, by both Chelsea and Manchester City, Guardiola is said to have been promised that all financial conditions would be met without problems by then Bayern president Uli Hoeness.

With Bayern carrying no debt, and able to appeal to their various corporate shareholders for particular investments, Guardiola's salary, estimated to be around £12m a year, was not central to discussions. Rather, it was footballing issues which the Catalan was keen to clear before taking the job.

Split: Guardiola and Kroos exchange words on the touchline at Old Trafford last season

Pass master: Kroos is now playing his trade in Real Madrid's midfield after a £20m summer transfer

Some have been well documented. Guardiola's desire to sign Mario Gotze from Borussia Dortmund was clear from the beginning, as was his ultimatum to the Bayern board with regard to Thiago Alcantara. 'Thiago or nothing', he is alleged to have said.

Perarnau's book, though, alleges that Guardiola also specified that Kroos should not be sold under any circumstances. The former Barcelona boss saw pass-master Kroos as a key ingredient to the project he wished to build in Bavaria.

In Guardiola's first season, Kroos was used only as part of a rotation policy with the likes of Thiago. This disgruntled the midfielder, and led to a famous touchline spat last winter. The player eventually demanded a far higher wage from Bayern, claiming he felt undervalued.

Injury headache: Guardiola will be without midfielder Thiago Alcantara (centre) for a lengthy period

That led to a whole six months of endless speculation as to whether Kroos would leave for the Premier League or La Liga, or if Bayern would cave to his demands. During negotiations with the midfielder, the narrative became clear that both Guardiola and director of sport Matthias Sammer were desperate to keep the player, while the board were happy to let him go.

'Kroos is a great guy, he loves the game. I would really like to hold onto him,' Guardiola said in April.

But with the board, led by Karl-Heinz Rummenigge, refusing point blank to raise Kroos' wages, the player was lost to Real Madrid after the World Cup. After his move, Kroos insisted that he had 'no problem whatsoever with Guardiola'.

Many had already seen the Kroos saga as a clear case of Guardiola being undermined by the board. With Perarnau's claim that Guardiola had demanded the player stay from the very beginning, that reading seems even more valid.

It remains to be seen how much Bayern will miss Kroos, but with both Thiago and Bastian Schweinsteiger set to miss a huge chunk of the early season through injury, his departure may prove to be a crucial development in Guardiola's second season in Munich.